The hydroformylation of olefin has been investigated on Co and Rh catalysts supported on active carbon at mild reaction conditions (403 K, 3.0 MPa) in the slurry phase reactor. Of the catalysts, Co/activated carbon catalysts showed excellent catalytic performance only in the alcoholic solvent, while Rh/activated carbon catalyst exhibited good activity in the nonpolar solvent. Rh/activated carbon catalyst showed stable activity for 14 h in the continuous feeding system, giving the total turnover number of 500. The effects of solvents were discussed in detail. The details of reaction conditions that affected the performance of the catalysts have been optimized. The higher catalytic activity was found at the lower temperature, about 403 K. About 20% of metal was dissolved from the support, a determination based on inductively coupled plasma (ICP) atomic emission spectrometry. The dissolved part was thought to deposit again on the support and work as active species as well.