One hundred and forty 21-day old Arbor Acres broiler chickens were used in a straight-run experimental design. The broilers were distributed in a completely randomized design among four treatment groups with seven replicates per treatment and five chickens per replicate. During the experiment period (21 to 42 days old), the chickens were fed iso-caloric and iso-nitrogenous diets containing four levels of dietary vegetable oils (DVO) (2.7, 4, 6 and 8%). During the period when chickens were 25–27, 31–33, and 38–40 days old, the chickens were exposed to heat stress for 4 hours a day (from 10.00am–2.00pm) at 34ºC and 70–75% relative humidity, and returned to normal house temperature afterwards. Feeding 8% DVO diet significantly increased body weight gain (BWG) compared to the other levels of DVO. Moreover, feed conversion ratio (FCR), protein conversion ratio (PCR), metabolizable energy conversion ratio (ECR) and European production efficiency factor (EPEF) were significantly enhanced due to feeding 8% DVO diet compared to 6% DVO diet. On the other hand, 6 and 4% DVO did not significantly differ from the control. Feeding 8% DVO diet significantly decreased plasma low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL) and lymphocytes. Feeding 8% DVO diet significantly increased mean corpscular volume (MCV) and mean corpscularhaemoglobin (MCH) compared to the control. Thus, it can be concluded that broiler chickens fed diet containing 8% DVO have showed tolerance to high ambient temperature (34ºC, 70–75% RH) during the period from 21–42 days old, as evidenced by increasing the productive performance, blood haematological and biochemical traits.