Abstract Three series of experiments were carried out to examine the effect of the size and type of aggregate on the size effect in the compressive strength of concrete from the standpoint that it may result from the geometrical heterogeneity of concrete system. According to the test results, the size effect in the concrete strength was closely related to the ratio of the diameter of specimen to that of aggregate (D/d). The compressive strength of river sand and gravel concrete and glass ball concrete increases with the increase of the size of specimen provided that the value of D/d is smaller than about 8.