Cubic 3C-SiC has been heteroepitaxially grown on the top Si layer of SIMOX (separation by implantation of oxygen) by chemical vapor deposition. One of the films is grown on a SIMOX that has a thick (similar to 200 nm) topmost Si layer while the other is deposited on a SIMOX with a thin (similar to 35 nm) topmost Si layer. The microstructure of two such films has been compared by cross-sectional and plan-view transmission electron microscopy. The defects in both films are predominantly planar in nature. The defect density of 3C-SiC epilayer deposited on the thin Si substrate is much lower than that on the thick Si substrate. The difference in the defect content of the two systems is tentatively attributed to the difference in compliance of the two substrates.