Abstract: Background and Objective: Studies on fish biology are essential for the sustainable management and conservation of fish biodiversity. This study observed the length-weight relationships and condition factors of common rudd fish, which give us some information about the ecological and nutritional conditions of fish. This information is crucial to fishery management. Materials and Methods: The length-weight relationship (LWR) and condition factors (K) of 270 Scardinius erythrophthalmus caught in Anzali Lagoon from April to August, 2015. Results: The total length and body weight of S. erythrophthalmus ranged from 94-179 mm and 11.98-98.5 g, respectively. A strong positive relationship was observed between length and weight (r = 0.96). There were no significant differences in the condition factors of males and females (p>0.05). The lowest K value was observed in April (K = 1.5±0.04) and the highest in July (K = 2.25±0.34). The sex ratio of 1.0 male: 1.15 female was observed, with no significant difference between males and females (p>0.05). Conclusion: In this study, the regression coefficient (b) in males and females was 3.35 and 3.29, respectively, more than 3.0, which means they have positive allometry growth in both sexes.