A study on the utilization of various levels of sunflower seed (SFS) in broiler diets was carried out in two experiments, each lasting 7 wk. Experimental diets contained 21, 19, and 17% CP for birds aged 1 to 3, 3 to 6, and 6 to 7 wk, respectively. Metabolizable energy was allowed to vary according to the level of SFS. In Experiment 1, 840 straight-run Hubbar broilers were randomly allocated to seven groups, each with four replicates. The first five groups were fed with diets containing 0, 15, 20, 25, or 30% SFS throughout the experiment; the last two groups were fed 0 and 10% SFS during the first period (1 to 3 wk of age) and 30 and 30% SFS during Weeks 3 to 7. In Experiment 2,810 straight-run Arbor Acres 707 broilers were randomly allocated to nine groups, each with three replicates. They were fed with 0, 30, 40, or 50% SFS throughout the experiment or 50% SFS during the first period and 30% in the last period (Weeks 3 to 7). At each level of SFS, the diets were either adjusted for lysine (Groups 3, 5, 7, or 9) to be equal to the control group (0% SFS, Group 1) or were not adjusted in lysine content (Groups 2, 4, 6, or 8). In the latter instances, lysine was supplemented at in the same level as in the control diet. The results indicated that birds fed SFS diets gained slightly more weight (1.6 to 7.1%) than the birds in the control group but the differences were significant only in Experiment 2. Feed consumption decreased with incresing SFS in the diet, which consequently improved the feed conversion ratio significantly. Liver and pancreas weight decreased, but abdominal plus visceral fat increased when SFS was fed. Differences among groups were not significant for mortality rate, hematocrit, hemoglobin, or serum cholesterol level. Neither production nor blood parameters measured were affected by the lysine level of the diets.