An HRTEM study was carried out for the orthoclase (Or65 Ab21 An11) taken from the augite-mozonite deposit in Wuliang, Shandong Province. The results showed that this material consists of superlattice with long period. The relationship between extended cell parameter and triclinic subcell parameter is as follows: a=atc, b = 8d010≈8btc c= ctc, β= βtc (tc represents the triclinic subcell).The orthoclase consists of a series of triclinic domains. It is the intermediate modulated structure formed in the process of phase transformation of potassic feldspar horn C2/m symmetry to C1 Symmetry through the ordering of Si and Al in the te-terahedra.