In order to study the formation law and mechanism of Voids in Coarse Aggregates(VCA) of binary coarse aggregates, the limestone coarse aggregates in grades A(19-31.5 mm), B(13.2-19 mm), C(9.5-13.2 mm) and D(4.75-9.5 mm) were combined in pairs to obtain 6 kinds of binary mixtures. The results of VCA of binary mixtures were obtained through vibration experiments. The experimental results showed that the VCA-values of binary aggregates presented concave curve with the increase of the amount of bigger size coarse aggregates, and there were valley values. But the changing rates of concave curves and valley values were different. With the increase of the ratio of geometric mean particle diameter between smaller coarse aggregates and coarse aggregates, the rate of concave curve changed more and the valley value was lower. By means of regression analysis, several effective statistical models of VCA of binary mixtures were established and a concept called filling effect coefficient was proposed to reflect the degree of smaller coarse aggregates filling in coarse aggregates. An effective statistical linear relationship between the filling effect coefficient and the ratio of geometric mean particle diameter was built, which showed that the size difference between smaller coarse aggregates and coarse aggregates was more, i.e. the ratio of geometric mean particle diameter was bigger, the effect of smaller coarse aggregates filling in coarse aggregates was better and VCA-value of binary mixtures was smaller. On those basis a unified regression model of binary mixtures was built and the formation mechanism of VCA of binary mixtures was revealed.