. River catfish (Mystus sp.) is a species commonly found in Blanakan river. This species is also consumed by community lived around Blanakan. The aim of this study is to investigate the effect of temporal variation of temperature and metal Zn on Mystus sp. size distribution. The fish samples and water were collected from river on early and end of June to represent temporal variation. Simultaneously, the water temperatures was also recorded. The fish sample was measured for it’s length and weight. The AAS (Atomic Absorption Spectroscopy) was used to analyse Zn in water samples. The results showed that there was a temperature decrease from 32.9 ⁰C to 30.8 ⁰C. The Zn was also decrease from 1.12 mg/l to 0.14 mg/l. The length of Mystus sp. increased from 13.27 cm to 14.00 cm. In contrast, the weight was reduced from 36.29 gr. to 32.88 gr. To conclude, the temperature and Zn temporal variation have effect on increasing length and reducing weight.