The RNA-Seq technique was used to investigate the differentially transcribed genes of the caterpillar fungus Cordyces militaris. It was calculated that >63% of total annotated genes were expressed, including >55% of putative pathogen-host interaction genes. Overall, the fruiting-body development and mycelium growth by C. militaris activated high expression levels of genes in association with translation and post-translational modifications. Otherwise, there were many differentially expressed genes when the fungus was grown on insect pupae to different stages or in liquid medium. In general, the transcriptional profiles were more similar with each other during fungal fruiting-body developmental stages than during growth in a liquid culture. Examination of differential gene expressions by Cordyceps militaris during fruiting-body developments on silkmoth pupae and growth in a liquid medium.