Through the study of some rock-forming minerals in intermediate-basic ig- neous rocks by means of UJniversal Stage, the author found that some mineral pairs occurred widely in regular intergrowth with each othea，e. g., olivine with orthopyroxene or clinopyoxene; or thopyroxene with clinopyroxene or hornb- lender; clinopyroxene with hornblende, and plagioclase with orthoclase etc. The cora-esponding crystal axes and planes of the pair of minerals in re- gular intergrowth are close to each other, and their opticaa elements appear to be orientated and symmetrical. The regular intergrowth of different minerals depends on their crystal structure, with the density of plane nets and the unit-cells playing the decisive role. The orientation in optical elements is due to their close relationship with their crystallographic elements, while biaxial their symmetry is due to their having the same symmetry type of indicatrix.