Ground high-oleic acid sunflower seed (HOASS) was included into a basal diet containing 90 g kg^-1 crude palm oil (PAO) to give levels of 0, 50, 100, 150 and 200 g kg^-1, which resulted in PAO:HOASS fat ratios of 100:0, 75:25, 50:50, 25:75 and 0:100. The five diets were isonitrogenous and isoenergetic and were evaluated using broiler chickens from 3 to 6 weeks of age. Apparent digestibilities of dietary fat, palmitic, stearic and total fatty acids and AME of fat improved as the ratio of PAO:HOASS fat decreased, the observed response being curvilinear. A positive synergism between both fat sources was detected as demonstrated by the significantly greater determined values for the apparent digestibilities of crude fat, total fatty acids, palmitic acid, and AME of the dietary fat mixture than the theoretical values calculated from the values for the single components of fat mixture. The equation for estimating the energy value of dietary mixed fats showed that ascribing the same energy value to all fat mixtures should not be recommended for formulation of diets.