This study described the reproductive behavior and the morphological development of the larvae and juvenile of the hedgehog seahorse (Hippocampus spinosissimus). Larvae released from male seahorse were reared for 30 days in the laboratory. At the birth, the small flap-like mouth and anus of the yolk-sac larvae were open, and the yolk was completely absorbed within 1 day. The size of newborn larvae ranged 7.01-13.19mm (mean $9.26pm1.9mm$, n=9) in TL (total length). The larvae had 15-16 pectoral fin rays and 19 dorsal fin rays, but 4 anal fin rays were not completely developed. The larval size ranged 13.38-14.38 mm (mean $13.96pm0.3mm$, n=6) in TL at 1 day after release, and they had 17 pectoral fin rays, 20 dorsal fin rays and 4 anal fin rays when the larvae reached the juvenile stage. Juvenile size ranged 14.98-16.42mm (mean $15.84pm0.5mm$, n=5) in TL at 3 days after release, and 1, 4, 6, 8 trunk spines extended quickly. Terminal part of the coronet became a four-forked wedge shape concurrently. Juvenile became 23.25mm (n=l) in TL after 15 days and all 11 trunk rings was connected completely. After 20 days, the juveniles were 24.66-28.65mm (mean $27.26pm1.6mm$, n=5) in TL, and many melanophores appeared in integument. In the eye lenses, 5-6 lusters were formed. After 30 days, the juveniles were 26.78-28.26 mm (mean $27.36pm0mm$, n=4)in TL and 8-9 lusters appeared on the eye lenses. Coronet terminal part developed to a five-forked crown-shape completely.