The graft density dependence of main chain stiffness in rod brushes composed of a flexible linear polystyrene (PS) main chain and poly(n-hexyl isocyanate) (PHIC) rod side chains has thoroughly been investigated by static light (LS) and small-angle Xray scattering (SAXS) measurements in tetrahydrofuran (THF) at 25 degrees C. A series of statistical graft copolymers having different graft PS densities (sigma) and graft chain lengths (PS-g-HIC-N-s-l(g), where N-s is the weight-averaged degree of polymerization of HIC and l(g) is the average distance (spacing) of the main chain between the side chain joints) were prepared by the nearly azeotrope radical copolymerizations of styrene (M-1) with PHIC macromonomers (M-2)(r(1) = 0.84 +/- 0.1 and r(2) = 1.1 +/- 0.3) in n-hexane or the bulk at 60 degrees C. The compositional heterogeneities of the graft copolymers were carefully characterized by the ratio of the UV to RI signal in the SEC traces and the comparison with those predicted from the theory by Stejskal and Kratochvil [Macromolecules 1987, 20, 2624], implying that they were sufficiently small enough to allow one to study their dimensional characterizations. The graft density dependence of the root-mean-square cross-section radius of gyration (< S-c(2)>(1/2)) and the z-averaged root-mean-square radius of gyration (< S-2 >(1/2)(z)) of PS-g-HIC-N-s-l(g) was studied and rationalized as the function of sigma and N-s. The sigma-dependence of < S-c(2)>(1/2) could be quantitatively described by the theory for the semiflexible comb whose main and side chains have different chain stiffness. The weight-averaged degree of polymerization of the main chain (N-M) dependence of < S-2 >(1/2)(z) was also quantitatively described by the cylindrical wormlike chain model. It was found that the change in the main chain stiffness (lambda(-1)(b)) resulting from the interactions among the side chains increases in proportion to the scaling law of lambda(-1)(b) proportional to N-s(1)sigma(1).