Abstract: K-feldspar megacrysts are very common in granitic rocks. They could be either microcline or monoclinic orthoclase，and their origins are still debated Microcline megacrysts，2 to 12cm long，are easily found in Fogang granitic complex，South China．They are usually euhedral in shape，common in simple Carlsbad twinning or grid-twinning，but quite often in cross interpenetration twinning．They are orientated with long-axes more or less parallel to the flow direction of the magma．Tiny minerals such as plagioclase，biotite．quartz etc, are poikilitically enclosed in them. In addition，the microcline megacrysts sometimes are surrounded by a thin plagioclase film forming mantled feldspar，and sometimes intergrown with quartz forming“pearl margin”．More petrographic features of subsolidus exsolution and oscillatory zoning can also be easily found under microscope X-ray powder diffraction analysis indicates that the triclinicity of the microcline megacrystis 0 963，tlo+ t1m= 0．970，t2o= t2m = 0．015，and therefore it is highly Si-Al ordered microcline. The microclines in groundmass of Fogang granitic complex are commonly in simple Carlsbad twinning or grid-twinning and exsolution texture，but with much smaller size and interstitial irregular shape． All the above mentioned petrolographic features show that the microcline megacxysts in Fogang granitic complex were crystallized from silicate melt．In order to further document the origin of the microcline megacrysts，we have also studied the microgranltoid enclaves in Fogang granitic complex as same microcline megacrysts have been found in these enclaves．These enclaves of quatz diorite or granodiorite composition with round shap,。needle-like apatite and biotite，ocular quartz and poikilitic texture, are typical magmatic enclaves formed by magma mixing．Therefore，the microcline megacrysts are really xenocrysts from the host magma． The detailed magmatic crystallization history could be addressed by using Swansons(1977) experimental diagram. At ca.850℃ ，plagioclase，biotite and K-feldspar crystallized simultaneously，but the K-feldspar has fllower nucleation density and higher growth rate than other minerals．Then plagioclase and biotite could be enclosed by the faster-growing K-feldspar，or simply were overtaken by the latter．However, as the mineral inclusions are usually concentrated at the centers of the K-feldspar, the outer parts of the K-feldspar were formed at a later stage．At ca .800℃ ，quartz started its crystallization, and its nucleation density was close to that of K-feldspar，but the growth rate of K-feldspar was much faster than that of quartz，and their intergrowth formed the“pearl margin”of the K-feldspar. At ca.600℃, even-shaped and small sized grainswere crystallized forming groundmass． The fact that the K-feldspar megacrysts and groundmass are microcline rather than orthoclase is significant for understanding the cooling history of the granitic complex. A possible mechanism is that the triclinic microcline was transferred from the monoclinic orthoclase during the slow cooling history．