Bigger animals tend to live longer than the small ones across species, but whether body size also has a robust relationship with survival within species remains to be determined. Here, the association between body size and fitness traits was examined through two food treatments (starvation and fed ad libitum) for both virgin and mated spider mites, Tetranychus urticae. The longevity of spider mites differed significantly across treatments, with feeding ad libitum increasing the survival of both males and females, mating decreasing male survival when starved and female survival when fed ad libitum. The body size of females but not of males increased with food. For each treatment, no clear correlations between body size and longevity were found. However, female fecundity was shown to have a positive relationship with body size. These results suggested that within species, for spider mites, there is no clear association between body size and longevity, but the fecundity increased with the body size, although this association is weak.