The Kocks-Mecking theory has spectacular success in the application of predicting the work hardening behavior of various metals. However, the current treatment of dislocations annihilation term in Kocks-Mecking theory relies on some fitting parameters, whose physical meaning needs to be clarified. In the present study the dislocations annihilation (dynamic recovery) will be investigated from a perspective of statistical mechanics. The velocity distribution function of dislocation segments to be annihilated is firstly assumed based on the principle of maximum entropy and then the number of microstates of dislocations system is evaluated. The statistical partition function of the dislocations system is also calculated. Finally, the expression of the energy barrier for the annihilation of dislocations segments is obtained based on thermodynamics. The sensitivity of the dislocations annihilation to strain rate and temperature is checked according to present model the model predictions are validated by experimental results.