The purpose of the present study was to determine differences in the grading abilities of the vertical jump between 2 different conditions. For this purpose, we evaluated grading abilities in a vertical jump with preferred timing (preferred condition) and a combination task involving simple reaction and vertical jump tests (quick condition). Fourteen healthy college students participated in the experiment. The participants were instructed to perform a vertical jump to 4 target levels, which were 25%, 50%, 75% and 100% of the maximum jump height, in each condition. The results showed that both jump height and relative jump height (%max) under each condition were shorter/lower under the quick condition than under the preferred condition. Correlation analysis demonstrated a significant correlation between the jump height and the maximum flexion angle of the knee, and the hip joint under both conditions. There was a significant correlation between jump height and maximum dorsal flexion angle of the ankle under the quick condition, but not under the preferred condition. From these results, it was suggested that combining a simple reaction task with a grading task might alter the behavioral properties during the grading task, as well as the relationship between subjective effort and jump height.