Abstract: The aim of the present study was evaluate the effect of finishing diets and the length of feeding on the quality of Chilean beef. A group of 75 steers were fed one of 5 finishing diets (n = 15 per diet): pasture for 90 days (P), oat grain plus pasture silage for 35 days (SO), oat grain plus pasture silage for 75 days (LO), wheat grain plus pasture silage for 35 days (SW) and wheat grain plus pasture silage for 75 days (LW). Physicochemical and sensory attributes and fatty acid composition of beef were determined. Beef from pasture-fed animals was more tener, darker, less red and with yellower fat than beef from grain-fed steers. The length of feeding with grain did not alter beef quality, except for the percentage of intramuscular fat which was higher and tenderness which was lower for the longer feeding period with wheat. Beef from pasture-fed steers had a more favourable fatty acid profile with higher levels of CLA and n-3 fatty acids and a lower n-6:n-3 fatty acid ratio than beef from grain-fed steers. Longer grain feeding decreased n-3 fatty acids and increased the n-6:n-3 ratio compared to the effect of shorter feeding, especially with the wheat diet.