Behaviour of Nuclear Force If nuclear force is attractive, then why the nucleons don't collide with each other? I think about this, but do not get any proper answer?
 A: This is because the strong force has an extremely short range. For the case of two protons, the electrostatic force of repulsion is enough to prevent them from getting close enough for the strong force to kick in, except at very high collision energies. Even then, the strong force attraction is not quite enough to bind them against their mutual electrostatic repulsion, and they do not stick together.
You can get two protons to bind by adding neutrons to the mix as "glue", which bind through the strong force but do not repel electrostatically since they are uncharged. This trick lets you build up large and stable nuclei containing up to 56 protons, so long as you include about the same number of neutrons to get the whole mess to stick together.
Note however that because the strong force that binds neutrons to protons is short-ranged, the chances that a neutron shot at a proton will bind to it is small because for that to happen, they have to strike each other essentially head-on. Since they are very small, this does not happen very often unless you are colliding huge numbers of neutrons and protons together.
A: You ask:

If nuclear force is attractive, then why the nucleons don't collide with each other? I think about this, but do not get any proper answer?

If by "the nucleons don't collide" you imply "and merge with each other"
I want to add to niels' answer that protons and neutrons are in the range where quantum mechanics is needed and sketch the quantum mechanical picture .
Electrons are attracted to protons by the electromagnetic force. Why does not the electron fall on the proton and neutralize it? It is one of the basic observations, the atomic spectra, that led to the need of the quantum mechanical theory. Because it is at the ground state in a stable orbital about the proton.
The case of the strong attraction between protons and neutrons is much more complicated, analogous to the complication of describing quantum mechanically the bonding of atoms and molecules to make matter as we know it. The proton  (and neutron) is made up of valence  quarks  and a sea of antiquarks and gluons, all held together by the quantum chromodynamic force, QCD,


Snapshot of a proton -- and imagine all of the quarks (up,down,and strange -- u,d,s), antiquarks (u,d,s with a bar on top), and gluons (g) zipping around near the speed of light, banging into each other, and appearing and disappearing. (M.Strassler 2010)

in a very complicated way.  The nuclear force that holds the protons and neutrons in a nucleus is a spill over of the QCD force.
A proton and a neutron are attracted by the spill over forces, but the "bag" of quarks is equivalent to the fixed ground state in the hydrogen atom that does not allow it to annihilate on the proton,  there can be only fixed valence quark representations that lead to stable hadrons  The theory that models this is QCD on the lattice.
The experimantally found hadrons have the valence quark content shown here, analogous to the quantum mechanically imposed constraints for the hydrogen atom energy levels.
So the nucleons in the nucleus are constrained to keep their identity, and merging cannot happen because of the way the valence quarks are absolutely  tied to the proton and neutron individually through QCD.
A: Niels' answer is good, but there is more to it.
Nuclei are surrounded by electrons, forming atoms. When atoms get close to each other, the electrons both atoms interact with both nuclei. The result is sometimes a chemical bond. The electrons are attracted to both nuclei, but the nuclei repel. The result is they stay close to each other, but apart.
Sometimes there is no bond. The electrons from each atom repel each other and the atoms just bounce off each other. The nuclei stay apart.
