Is temperature of 1 Kelvin equivalent to 1 eV in natural unit? We know that the Boltzmann's constant, $k_B$=8.617 $\times$ $10^{-5}$ eV/K. Now in the natural unit, $k_B=1$.
So can I say, in the natural unit, 1 K temperature is equivalent to 1 eV in energy? 300 K is equivalent to 300 eV?
Or am I missing something?
 A: No.  The situation is similar to the situation in special relativity where we use units in which $c = 1$ instead of $c = 3.00 \times 10^8 \text{ m/s}$.  Using $c = 1$ does not mean that 1 meter is now equivalent to 1 second.  It means that we are defining our unit for distance to be the distance traveled by light in 1 second.  In other words, "1 second of distance" is the distance traveled by light in 1 second of time.
In the case you're interested in, where we use "natural units" in which $k_B = 1$, temperatures are measured in units of energy.  In other words, we could say that a system has a temperature of 1 eV, or 300 eV;  but these are not equivalent to 1 K or 300 K any more than "1 second of distance" is equivalent to 1 meter.  Rather, we are defining our temperature scale as something like, "if the average KE of an atom in a monatomic ideal gas is 3 eV, then the temperature of the gas is defined to be 2 eV."  The change from 2 to 3 is because of the $\frac32$ factor in the equipartition theorem for the ideal gas:
$$
\langle E \rangle = \frac{3}{2} T
$$
if $k = 1$.
(Finally, a nitpick on your question:  in conventional units, $k_B = 8.617 \times 10^{-5} \text{ eV/K}$, not $8.617 \times 10^{-5} \text{ eV}$.  This may have been a typo, but note that the correct units make it clearer that Boltzmann's constant is a "conversion factor" between temperature and energy.)
