A workload characteristic on a parallel computer depends on an administration policy or a user community for the computer system. An administrator of a parallel computer system needs to select an appropriate scheduling algorithm that schedules multiple jobs on the computer system efficiently. The goal of the work presented in this paper is to investigate mechanisms how job size characteristics affect job scheduling performance. For this goal, this paper evaluates the performance of job scheduling algorithms under various workload models, each of which has a certain characteristic related to the number of processors requested by a job, and analyzes the mechanism for job size characteristics that affect job scheduling performance significantly in the evaluation. The results showed that: (1) most scheduling algorithms classified into the first-fit scheduling showed best performance and were not affected by job size characteristics, (2) certain job size characteristics affected performance of priority scheduling significantly. The analysis of the results showed that the LJF algorithm, which dispatched the largest job first, would perfectly pack jobs to idle processors at high load, where all jobs requested powerof- two processors and the number of processors on a parallel computer was power-of-two.