Vinyl chloride-diallyl phthalate (VC-DAP) suspension copolymerization was carried out in a 5-L autoclave and 200-mL stainless steel vessel at 45 degrees C. The apparent reactivity ratios of VC-DAP suspension copolymerization system were calculated as r(VC) = 0.77 and r(DAP) = 0.37. It shows that VC-DAP copolymer contains no gel when the feed concentration of DAP (f(2)(0)) is lower than a critical concentration (f(cr), inside the range of 0.466-0.493 mmol/mol VC at 80-85% conversion), the polymerization degree (DP) of copolymer increases with the increase of f(2)(0) and conversion. VC-DAP copolymer is composed of gel and sol fractions when f(2)(0) is larger than f(cr). The DP of sol fraction decreases as f(2)(0) increases, but the gel content and the crosslinking density of gel increase. The gel content also increases as conversion increases. The results also show that the index of polydispersity of molecular weight of sol changes with f(2)(0), a maximum value appears when f(2)(0) is close to f(cr).