Abstract: Early embryology of the Lampetra japonica has been studied based largely on external morphology and histology. In current study, ecological experiment was conducted to investigate the allometric growth of L. japonica yolk-sac larvae. The results indicated the ovulated egg was creamy-white with the oval shape. The cleavage of zygote was holoblastic cleavage. The early embryology of L. japonica had been characterized into cleavage, blastula, gastrula, neural plate and groove, head, prehatching and hatching. The embryo breaks through the fertilization membrane at 11—12 day-post-fertilization. Newly-hatched larvae were (3.41±0.24) mm in total length and about 0.0006 g in weight. Study of the early embryology ofL. japonica may provide the basis for understanding the origin and evolution development of vertebrates. During yolk-sac stage, head and tail length exhibited positive allometric growth, and trunk exhibited negative allometric growth. The inflection point was at 7 day-post-fertilization for both head and tail length. The head and tail length exhibited positive allometric growth before and after the growth inflection point and the growth was higher post the inflection point. In terms of the dimensions of organs in head, snout length, prebranchial length and branchial length exhibited positive allometric growth. The inflection point appeared at 9 day-post-hatch for snout length and at 8 day-post-hatch for branchial length, respectively. The inflection point of disc length occurred at 3 day-post-hatch, and the disc length exhibited isometric allometric growth to the inflection point, then became positive allometric growth. The eye length was characterized by isometric allometric growth and had no inflection point. The postocular length exhibited negative allometric growth. The cloacal slit length exhibited positive allometric increase and hads no inflection point. The allometric growth pattern of L. japonica yolk-sac larvae enabled rapid development of the organisms which has important function to adapt to the early life stage and increases their survival rate.