(1). This paper involves some of the concepts of modulated structures and complex structures of alkali feldspars and their experimental methods. Complex structures such as density modulation, periodic twinning and cohe- rent exsolution etc. were found in alkali feldspars of Hanmuoba basalt, Zhangjia- kou, Hebei Province, and in the volcanic elastic rocks of northeastern Jiangxi Province. X-ray single crystal analysis methods such as precession method, standard orientation oscillation, Weissenberg method and transmission electron microscopy (TEM) were employed to reveal these complex structures of alkali feldspars, and the techniques are outlined in this paper. (2). the Hannuoba Basalt is located in the northwestern part of Zhangjiakou, Hebei province. The distribution of basalt is controlled mainly by the two fractures in the area. Several kinds of deep-source、inclusipns and megacrysts such as feldspars and pyroxenes were found in the basalt. For about sixty years，all the feldspar megacrysts found in this area had been regarded as anoithoclases.On the basis of anaIysis of compositions, optical properties and X-ray diffraction patterns， oligoclase-andensine megacrysts were first discovered by the author in Zhangjiawan Tholeiite Grater in the area. The structure of an andesine megacryst in it has been determined. The least-squres refinement led to a final discrepancy index R0.096.The bond lisngths of T-O tetrahedrons and A1 occupancy indicate that Al/Si distribution is disordered. (3) The paper is concerned with the anorthoclase rnegacrysts collected from Damaping Volcanic Cone, Zhangjiawan Lava Sheet and Taoyaowan Pipe in Han- nuoba Basalt, which are all alkali basalt. The study is focused on the complex structures of the anorthoclase megacrysts and was carried out mainly by means of single-crystal diffraction in addition to the powder X-ray diffraction, chemical analysis and optical study. The anorthoclase megacrysts from Damaping are cha- racterized by their simpler structures. Only albite law or pericline law twinning can be found on the ;single diffraction patterns. The same patterns of anorthoclase megacrysts from Taoyaowan Pipe are characterized by bond satellite diffractio.ns, which are . caused by density modulation structures. The direction of its wave tran- smition is nearly parallel to the normal of (10 0 1) and the length is about 100A.Several kinds of complex structure diffractions including coherent decom- position (cryptoperthite) and long-period albite twinaing (transverse modula- tion structure) were found on the patterns of, the anorthoclase megacrysts from Zhangjiawan Lava Sheet. The period of the transverse modulation is 17 to 18b. An anorthoclase crystal structure has been determined. The least-squares refinment}led to avfinal discrepancy index R=0.072. The bond lengths of T-O tetrahedron indicate that the structure is almost topochemically monoclinic sym- metre.