The morphological features of spermafids at different stages, and the structural dynamic changes of organelle during spermatogenesis of Octopus tankahkeei were investigated by transmission electron microscope. Spermatogonium is elliptic in shape with large nucleus. Less mitochondria and Golgi bodies in cytoplasm of spermatogonium. The primary spermatocyte is round or oval in shape, with more mitochondria in cytoplasm. The endoplasmic reticulum is more developed and is rope-like in shape, and the Golgi bodies accumulated and the centriole is also seen in the cytoplasm in the primary spermatocyte. The secondary spematocyte is round, elliptic or irregular in morphology. The cristae in mitochondria in the secondary spermatocyte are well developed, and the matrix of mitochondria is rich; also the electron density materials were accumulated in Golgi vacuoles and the ＂9 ＋ 2＂ arrangements of axoneme are extended from centriole at this stage. The spermiogenesis can be divided into 6 continuous steps based on the morphological changes of acrosome, nucleus and mitochondria. Three main events happen during spermiogenesis. （ 1 ） The chromatin in spermafid undergoes a process of granulation, fibrosis, and stratification, and finally is highly condensed with high electron density. At the same time, the morphology of nucleus changes gradually from ellipse, and long ellipse to slim column. Endonuclear channel was formed by the concave of the posterior nucleus. The coagulation of chromatin and the pressure posed by peri-nuclear microtubules help the formation of the morphology of the nucleus. （2） Golgi vacuoles accumulate density materials, fuse into acrosoml vesicle, and finally, the pre-acrosome becomes an aiguiUe-like acrosome. The striations and spikes can be seen on acrosome. Endoplasmic reticulum also participates in acrosome biogenesis. （3） Mitochondria move posteriorly to the nucleus and form the chondriosomal mantle of the tail. The formation of the chondriosomal mantle is the biological adaption to internal fertilization of this species. Because of the long chondriosomal mantle, the sperm of Octopus tankahkeei supply more energy than the sperm of bivalves and Archaeogastropoda, the later undergo exterior fertilization. The features of six stages during spermiogenesis are summarized as follows： From stage Ⅰ to stage Ⅱ, the chromatin transforms from conglomeration to granulation, the later which distribute evenly in the nucleus, and also, the peri-nuclear microtubules appear, the posterior nuclear pocket invaginates deeply, the nucleus elongates, and the acrosomal vesicle heaves up; at the third stage, the chromatin is flocculent in shape. Endonuclear channels are formed; at the fourth stage, the chromatin is long fibroid in shape, acrosomal cone is formed; at the fifth stage, the chromatin become stratification, acrosomal vesicle is long cystiform in shape; at the sixth stage, the nucleus is long columned in shape, with condensed chromatin inside, acrosomal vesicle has developed to long cone acrosome and the chondriosomal mantle is formed at mid-piece of tail.