Abstract: The void content in composites is determined by comparing measured composite depsity with theoretical density.The theoretical composite density is calculated from each component content and. its density.For multiphase composites,there are lots of parameters(various component contents and densities) included.,and every parameter is associated with certain error, So the total error resulting from the combination of these parameters is considerable.The more the number of components in composite the larger the error of void content.This paper analyzed the error of void content of a three-phase composite (glassfiber/carbon fiber/epoxy resin).The density of the resin in the composite is assumed to be the same as it is in a cast mass.It is,however,not strictly, correct.This paper described two methods of making cast resin. One is casted directly and the other is undergone vacuum treatment.The densities obtained from these two methods are not same.The former should be the minimum value of density of the resin matrix in composite and the latter should be the maximum value of the resin matrix density.There is no realistic way to determine the real value of the resin matrix density.It is the most serious error source in calculating void content in fiber rein-forced plastics to replace the density of resin matrix in composite with the density of bulk cast resin.This paper also indicated that as the number of components got more and/or as the void content got lower,the error in densities and contents of Components got more important.