The length-weight relationship (LWR) and condition factors (CF) of 1200 of the oriental river prawn Macrobrachium nipponense, collected from the Siah Darvishan river from January to December 2015 were studied. The total length and body weight of M. nipponense ranged from 1.23-9.3 cm and 0.1-11.7 g, respectively. A strong positive relationship was observed between the prawn length and weight (r= 0.96). There were significant differences in the condition factors of males and females (P < 0.05). The lowest CF value was observed in April (CF= 0.81 ± 0.04), while the highest was recorded in August (CF= 1.30 ± 0.34). The sex ratio of 1.0 male: 1.98 female was observed, which deviated significantly from the theoretical one male: one female ratio, indicating that males were significantly more than females (P < 0.05). In this study, the regression coefficient (b) was more than 3.0, a value that indicates positive allometry.