A transmission electron microscopy (TEM) study was made to investigate the mechanism of isothermal crystallization from melt for syndiotactic polypropylene (sPP) with a syndiotactic pentad fraction of 0.92. The TEM morphologies of RuO4-stained and ultrathin-sectioned specimens revealed a clear assignment of lamellar crystals with different crystal thicknesses to their corresponding melting temperatures and types of crystal structures. It has been found that isolated, single crystal lamellae with cell III structure were grown at 140 degrees C by a plausible crystal growth mechanism with regime I. Fringed-micellar type of lamellae with cell II structure as thin as c. 4 nm were formed in the interstices of the thick chain-folded lamellae by the subsequent quenching.