Diamonds were reproductively formed from graphite in the presence of Ag2CO3 and Ag2O at 7.7 GPa and 1500-2000 degrees C for 0.5-27 h. The complete conversion from graphite to diamond was confirmed in the samples obtained at temperatures above 1800 degrees C for 0.5-2 h, but no diamond was detected in the system of Ag and graphite under these conditions. To make clear the mechanism of diamond formation from graphite-Ag2CO3 system, decomposition temperatures of Ag2CO3 and Ag2O were investigated at 7.7 GPa and it was found that they were 1000 and 1200 degrees C, respectively. Then, the decomposition products of Ag, CO2 and O-2 were present instead of the carbonate in the diamond-forming condition. Since O-2 should react with graphite to form CO, in the HP-HT condition, our experimental results strongly suggest that fluid CO, has a strong catalytic action for the formation of diamond from graphite.