Two dominant parasitoids, Platygaster matsutama Yoshida and Inostemma seoulis Ko, often coexist in the host-parasitoid community of the pine needle gall midge, Thecodiplosis japonensis Uchida et Inouye. Subtle differences in parasitoid phenology appear to play a key role in reducing interspecific competition, but the specific thermal conditions for development of each species have not been defined. We examined the thermal biology of two sympatric parasitoids during postdiapause development at seven constant temperatures (12, 15, 18, 21, 24, 27, and $30^{circ}C$) to determine species-specific developmental parameters. The lower developmental threshold was estimated to be $4.2^{circ}C$ and $8.4^{circ}C$, and the thermal constant was 741.2 and 946.1 degree days (DD) for P. matsutama and I. seoulis, respectively, using the linear model. The nonlinear model determined that the optimal temperature was 24.8 and $26.5^{circ}C$ for P. matsutama and I. seoulis, respectively, but with an upper threshold temperature ($30^{circ}C$) that was the same for both species. The development rate of P. matsutama was higher than that of I. seoulis over the entire thermal range, and the difference was highest at $21.5^{circ}C$. Despite the lower developmental rate, I. seoulis was more heat tolerant and its optimal temperature was closer to that of host insect than those of P. matsutama. The results showed clear differences in thermal biology between P. matsutama and I. seoulis. Potential implications are discussed with respect to coexistence of two parasitoids on a single host and biological control of T. japonensis.