This experiment studied the effect of phytase and processing (heat treatment, grind size) on performance and nutrient utilization of broilers fed diets from 1 to 21 d that included hardred spring (HRS) and durum wheat with three grind sizes (470, 560, 630 µm) with or without heat treatment (oven dry at 80°C for 15 h) before supplementation with phytase (Natuphos® 0 or 600 FTU kg-1). All 24 diets contained 0.5 g Avizyme 1302 kg-1 and were formulated (80% wheat, 20% basal diet) to supply nutrients necessary to meet the requirements of broiler chicks, except for a low level of nonphytate phosphorus (P, 2.7 g kg-1). The medium ground HRS-based diet produced significantly better feed conversion ratio (FCR) and body weights compared with the other two grind sizes. Heat treatment significantly improved feed conversion ratio and apparent metabolizable energy (AME), protein, and P digestibility of diets. It also significantly decreased feed intake, but produced no differences in 21 d body weight. Supplementation with phytase significantly improved AME, protein, and P digestibility of test diets. The AME of diets was influenced by wheat type, processing and grind size. Phytase increased N and P digestibility. Improvements in N and P digestibility by phytase and heat treatment were dependent on wheat type and grind size.The present results suggest an advantage of both phytase and xylanase in durum- and HRS-based diets. Heat processing destroyed endogenous phytase and xylanase in wheat-based diets, and increased the levels of soluble non-starch polysaccharides. Combined phytase, heat treatment, and grind size may synergistically improve N and P digestibility. Key words: Wheat source, grind size, heat treatment, phytase, digestibility, broiler