Abstract: In general, lazurites are sweepingly described as symmetr ically cubic and optically isotropic. It was observed that, however, some lazuri tes are of optical anomaly, i.e. they are optically anisotropic. By means of the TEM investigation on lazurite from Baikal, Russia, a diversified variance in la zurite structure is observed, and combining with these diversified structures, t he cause of optical anomaly of lazurite is discussed in this paper as follows: ( 1) The disordered structure of lazurite has a space group of P43n, and consequently it is optically isotropic; (2) The structural ordering, which r esults from that the centers and corners of cubic cells of lazurite are orderly occupied by two different clusters respectively, would lead to an ordered supers trcture. The latter keeps cubic symmetry (but space group becomes P23) and o riginal cell size. Therefore it is still optically isotropic; (3) In the lazurit e studied, the authors discoveried an ordered superstructure which is one-dime ntionally commensurate-modulated along ［110］ direction with a repeated period of 4d110. This kind of structural modulation makes crystal to b ecome optically anisotropic, because its symmetry is non-cubic; (4) In lazurite the structural modulation resulting from partial ordering is usually incommens urate. It takes place frequently along one or several directions of <110>, <100> and/or <112>etc. Such modulated structure is also generally symmetrically non -cubic and optically anisotropic; (5) However, whether the modulation is commen surate or incommensurate, the structure will be symmetrically modulated-cubic a nd optically isotropic if the modulation concurred along all the symmetrically e quivalent directions. Based on a way proposed by the present authors, the existe nce of suc h modulated-cubic structure in the lazurite studied has been confirmed.