A probability associated with the population fitness is used in a genetic algorithm (GA) to terminate the evolution. The theoretically probabilistic derivation of population fitness reveals that the probability is inversely proportional to the individual variation and directly proportional to the evolving error between the average individual and the global optimum of the objective function. Based on the probability, GA switches its operation mode between the genetic operation and the population regeneration. A modified genetic algorithm with a termination strategy is proposed to find the global optima of five objective functions and thus validate the proposed probability of population fitness.