Abstract: The green sunfish (Lepomis cyanellus), a new invasive fish in the Ta-pieh Mountain Area, was found in the tributary of Pihe River in April, 2015. A field investigation was conducted with 78 green sunfish caught from Heigou Creek in July 2015 to measure the fish's morphological characteristics. The results revealed a high similarity between these fish but with certain morphological differences. Rather than gender, age might be the main reason for this difference. The most significant differentiation was in male but not female fish at the age of 1-2 years old, which might explain by the breeding habits of males. Some morphological characteristics of males, such as the head indexes (D1-2, D2-3, interorbital width, postorbital head length, head length), the near anal fin indexes (D5-7, D5-8), the dorsal fin coxal (D6-8) and caudal peduncle width (D9-10), were larger than those of the females. Furthermore, the females and the fish at 0 years old were longer and higher caudal peduncle indexes (D8-10, D8-9) than those of the male fish at 1 years, which may adapt to live in moving water. Due to these morphological characteristics, the males are apt to live in the deep pools of streams, while the females and fish at 0 years old are likely to occupy the lotic habitats (run and riffle), which has brought great ecologically potential dangers to the fishery in Ta-pieh Mountains.