Porous carbon monolith was used as substrate for microwave plasma chemical vapor deposition. The depositing time was set as 1 hour, 3hours, and 5 hours respectively. The deposited samples were characterized with Raman spectroscope and scanning electron microscope. It was proved that diamond was formed on the surface of all the samples. However, the surface morphology and phase composition of the chemical vapor deposition layer changed with depositing time. It was deduced that the porous carbon monolith was a carbon resource, and it gave out small molecules containing carbon element during the deposition. This changed the ratio of carbon to hydrogen in the growing atmosphere, and correspondingly influenced the growth of carbon on the substrate. With a low ratio of carbon to hydrogen, mainly diamond grew. Or else, other types of carbon would grow on the surface of the porous carbon monolith substrate.