The objective of this study was to evaluate the effects of replacing forage source with palm leaf according to undigested neutral detergent fiber (uNDF) on the performance, nutrients digestibility and feeding behavior of fattening lambs. twenty four Afshari growing male sheep with a mean body weight of 33 ± 3 kg were used in a completely randomized design with 3 treatments during a 60 days period. The experimental diets consisted of alfalfa hay and wheat straw, in which the palm leaf according to undigested fiber (uNDF) replaced forage ration in the form of: 1) Control diet containing 100% forage uNDF from alfalfa hay and straw without palm leaf. 2) Diet containing 50% of forage uNDF alfalfa hay and straw and 50% palm leaf (50% PL). 3) Diet containing 100% forage undigested fiber of palm leaf (100% PL). The diets were balanced according to energy, protein and neutral detergent fiber. Geometric mean for control diets, 50% and 100% undigested fiber of palm leaf were 2.09, 2.30 and 2.36 mm, respectively. Replacing palm leaf with alfalfa hay resulted in a linear increase of residual particles on the 1.18 mm sieve, but no differences observed in the residual particles on the 19 mm and 8 mm sieve. The amount of neutral detergent fibers and the physical effective fibers increased with increasing levels of palm leaf. Lambs fed with palm leaf consumption more than 1.18 mm sieve and consumption less than the try compared to the control diet. The organic matter and NDF digestibility decreased linearly with increasing palm leaf (P 0.05). However, no difference was observed in dry matter digestibility between experimental diets. Replacing alfalfa hay and straw with palm leaf did not have any effect on nutrient intake. The final weight, daily weight gain and feed conversion ratio were not affected by treatments. In terms of feeding behavior, total eating time and eating rate, rumination, ruminal rate and rest were not affected by experimental treatments (P 0.05).There were no significant difference in rumen pH and ammonia nitrogen among treatment. Blood urea nitrogen, glucose, albumin, total protein and enzyme alanine transferase were not affected by treatments. Economically, the price per kilogram of diet containing the palm leaf was less than the diet without the palm leaf. In general, the results of this experiment showed that the consumption of concentrate with 50% or 100% of forage undigested NDF from palm leaf could be used to feed lambs without negatively affecting the performance of lambs. Keywords: palm leaf, uNDF, Afshar sheep, performance