An enzyme assay and a feeding trial were conducted to assess the activity and efficacy of xylanases from Trichoderma sp and Thermomyces sp in broiler chicken. DNS assay revealed that the xylanase activity of the three preparations were 30,873, 7,545, and 2,507 U/g solid, respectively. In the feeding trial, each of the three xylanases was added at 0, 200, 500 and 1,000 U/kg feeds to wheatbased diets. Results showed that xylanase was active in the crop, jejunum, and ileum but not in the gizzard. The specific viscosity of digesta from the jejunum and ileum of broilers fed wheat-based diets supplemented with xylanase significantly decreased up to the 500 U/kg feed level. Moreover, xylanase supplementation significantly improved apparent metabolizable energy (AME) of starter diet and live weight gain and feed conversion ratio of broilers. Significant interaction between preparation and level of xylanase activity was observed only on the AME of starter diet. The results further indicated that the xylanase preparation had similar effect on growth performance parameters even with increased level of enzyme supplementation.