To explore the effect of between breeding temperature on the endosymbiotic bacteria in Aphis fabae Scopoli and body size of their hosts, the changes in the abundance of mycetocytes and host body size under different temperatures were observed in the laboratory. The results showed that the effect of temperature on the abundance of mycetocytes was dependent on the developmental stage of aphids. The abundance of mycetocytes in the 1st instar nymphs of the aphid showed no difference at different temperatures except at 25℃ and 35℃, but showed significant difference in the other life stages of the aphid at different temperatures, and a negative correlation between the abundance of mycetocytes and temperature was found. Temperature also affected, to some extent, the trend of the abundance of mycetocytes following the development of aphids. The abundance of mycetocytes increased with the development from nymph to adult before larviposition in the aphid at 15℃，20℃ and 25℃, but culminated at the 3rd instar and thereafter declined at higher temperatures (30℃ and 35℃). Aphid body size in all stages except for the 1st instar declined with the increase of temperature but increased with the abundance of mycetocytes at all temperatures except at 35℃. It is tentatively hypothesized that temperature affects aphid body size via influence on the abundance of mycetocytes.