Background and objective: Globally, the growing demand for poultry products cannot be overemphasized. Hence, this study was conducted to determine the effect of graded levels of aflatoxin B1 on growth parameters of broiler chickens. Materials and methods: Thirty day-old broilers were used for this study. The birds were house in pens and allowed free access to feed and water. At two weeks of age, the birds were divided into six groups. Group A served as control while groups B, C, D, E and F received 0.25, 0.5, 1.0, 2.0 and 4.0mg/kg body weight of crude aflatoxin B1 orally. Data were obtained on body weight and linear body measurements. Data obtained were analyzed using analysis of Variance (ANOVA) test at 5% level of significance. Results: Result obtained revealed that there was a significant reduction in the body weight gain of birds treated with aflatoxin B1 when compared with the control. More so, linear body measurements: shank length, keel length, thigh length, drum stick length, body length, body width and height, significantly reduced in aflatoxin treated birds when compared with the control. The effect of aflatoxin B1 was dose dependent. Conclusion: Findings of this study clearly show that aflatoxin B1 has significant negative effect on growth parameters in broiler chickens in a dose dependent manner.