Technical Abstract: A major gene controlling leaf color (lg for light green versus normal green) of sunflower was identified and mapped in an F2 population derived from a cross between two breeding lines. All 20 F1 plants from the cross had normal green leaf color and the segregation of 172 plants in an F2 population fit the monogenic ratio of 130 green-leafed to 42 light green-leafed plants (Chi squared(3:1)=0.03, p>0.9). The greenness degree of leaf color of individual F2 plants was measured with a hand-held chlorophyll content meter. The greenness degree varied from 30.2 to 41.6 and 15.3 to 27.1 within the normal green- and light green-leafed groups, respectively. These results indicated that leaf color is a monogenic trait with normal green dominant over light green. Measurement of chlorophyll with a spectrophotometer revealed that the difference in leaf color resulted from chlorophyll content variation among the plants. The contents of chlorophyll a, chlorophyll b, and total chlorophyll in the green-leafed lines were 71.9%, 115.0%, and 79.4% higher than those in the light green-leafed lines, respectively. Genetic mapping with molecular markers positioned the lg gene to linkage group 10 of sunflower. A previously mapped SSR marker, ORS 595, cosegregated with the lg locus, and a TRAP marker, B26P17ga5-300, was linked to the lg locus with a genetic distance of 4.2 cM. The markers tightly linked to the locus will serve as the starting point to isolate this gene through map-based cloning.