Chub mackerel (Scomber japonicus) is a straddling fish stock commonly utilizedby Taiwan. We carried out the present reproductive biology study for chub mackerel in the southwest Taiwan.We collected chub mackerel samples monthly from the catch in the southwest waters off Taiwan during November, 2020- October 2021. A total of 692 valid samples were used in this study, including 305 females, 387males. These samples were used to conduct reproduction and maturation for the stock. The major results were summarized as follow: The Gonosomatic Index (GSI) indicated that spawning period of chub mackerel were between November and April. Mean while, the gonad histology indicated that the female chub mackerel reached a high maturity rate in February to March, which implied that the spawning period, peaked around that time.Based on a logistic model, sizes at 50% of maturity were 213.67 and 215.59 cm in FL for female and male specimens, respectively. The mean trophic level (MTL), contribute tropic level (CTL) and fishing-in-balance (FiB) were estimated by the landings data from the Linyuan Fishermans Association in Kaohsiung. In this study, the Taiwanese-seine by the MTL 3.63~3.97 from 2012 to 2020.The FiB value is greater than 0,the Contribute trophic level (CTL) shows that target fish species for the main catch include Mene maculate(TL = 3.5)、Katsu wonus pelamis (TL = 4.3)、Auxis rochei(TL = 4.1) and Thunnus albacares(TL = 4.3).