# Why we not consider pion mass in nucleus

For the nuclear force there is existance of pion is important.why do not consider it in mass of nucleus.

Pions are the excitations of the pion-field. If you excite a field with the right frequency you get a "stable excitation", one that can propagate through space without changing it's shape. This happens when the norm of the momentum four-vector $m^2 = p_\mu p^\mu$ is equal to the mass of the field $m = m_\pi$. Such an excitation is called a real (or on-shell) particle, and is what you commonly imagine when you hear "pion".
If you however do not hit the right frequency, your excitation will quickly decay. It's like when you hit a guitar string but don't get it to ring (it will just make a bzz sound). This happens when $m \neq m_\pi$, and such a particle is called a virtual (or off-shell) particle. Since it does not travel through space-time like a real particle, the $m$ obtained in this way does not correspond to a mass in the kinetic sense.