This study aimed to evaluate the morphometric parameters of the ruminal papillae, and intestinal villi of lambs fed diets containing Mimosa tenuiflora (Willd.) hay replacing Buffel grass (Cenchrus ciliaris L.) hay. Twenty-eight Santa In & ecirc;s male lambs with initial body weight of 20.3 +/- 1.49 kg were used, being distributed in a completely randomized design with four experimental treatments (0, 333, 670, and 1000 g/kg of Mimosa tenuiflora hay). The width of the ruminal papillae was not affected (P > 0.05), while their area decreased linearly (P = 0.01) as the level of M. tenuiflora hay increased. In the duodenum, the height (P < 0.0001) and surface area (P = 0.01) of the villi decreased linearly with increasing M. tenuiflora level, while the width (P = 0.0001) and area (P = 0.03) decreased quadratically, reaching minimum estimated values of 132.95 and 50,625.53 m at 436 and 575 g/kg of M. tenuiflora hay, respectively, when used as a replacement for Buffel grass hay. There was a quadratic increase in the height of the jejunum villi (P < 0.0001) with a maximum value of 623.22 m at the level of 333 g/kg, while the width quadratically decreased with a minimum value of 143.33 mu m at the level of 526 g/kg and the area linearly decreased (P = 0.02). The height of the ileum villi (P = 0.0003) as well as the depth of the crypts in all segments (P < 0.001) linearly decreased. Therefore, it is suggested the substitution of C. ciliaris hay by M. tenuiflora hay up to 333 g/kg DM of the diet not affecting ruminal and intestinal morphometric parameters.