The ultrastructures of spermatogenesis and mature spermatozoa in Sepia lycidas were determined by scanning electron microscopy and transmission electron microscopy. The results show that there are five developmental stages of spermatogenesis: spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, and mature spermatozoon. The spermatid undergoes five phases (Ⅰ through Ⅴ) for differentiation, and the phase Ⅱ can be further divided into early and late periods. During spermameiogenesis, the nucleus undergoes a process of lateral contraction and longitudinal elongation, making it convert from a round or oval shape to an irregular spindle-like shape, and finally to a slightly curved long cylinder shape; the size of the nucleus reduces from 6.27 μm × 5.84 μm to 5.75 μm × 1.53 μm. The chromatin in the nucleus forms flocculent blocks, dense granules, fine fibers, crude fibers, and lamellae, and condenses into a region with a high electron density. The morphology of acrosome is also changed, from a round shape with a diameter of 1.02 μm, to helmet-like, conical, backward "U"-like, and finally to bullet-like with a length of 1.71 μm and a maximum width of 0.91 μm. The vacuole-shaped mitochondria experience migration and fusion, and they exhibit an oval shape with abundant cristae, and form a mitochondrial spur that incompletely surround the flagellum in the middle piece. The mature spermatozoa are 101.28 μm long, and consist of a head and a tail. The head shows a long pepper shape, approximately 7.73 μm in length and 1.15 μm in width, and it consists of an acrosome and a nucleus. The arosome resembles a bullet with a length of 1.71 μm and a width of 0.91 μm, and the nucleus is slightly curved and cylindrical, and is approximately 5.75 μm long and 1.53 μm wide. The 93.18 μm long tail is slender with a typical "9+2" structure, and it consists of middle, principal, and end pieces.