Conformational properties of cylindrical rod brushes consisting of a flexible polystyrene main chain and poly(n-hexyl isocyanate) (PHIC) rod side chains have thoroughly been studied by static light and small-angle X-ray scattering (SAXS) in tetrahydrofuran (THF) at 25 degrees C. These rod brushes were prepared by radical homopolymerization of 4-vinylbenzyloxy-ended PHIC macromonomers (1) (VB-HIC-N-s, where N-s is the weight-averaged degree of polymerization of HIC and in a range from 21 to 80) in n-hexane at 60 degrees C. The mean-square cross-sectional radius of gyration (R-c(2))(0)) of the brush at an infinite dilution is determined by SAXS measurement and rationalized as a function of N-s. The experimental value of (R-c(2))(0) gradually increases with N-s but above Ns higher than 20 increases to follow the scaling law of (R-c(2))(0) proportional to N-s(0.87). The experimental N-s dependence of (R-c(2))(0) is compared to that from the wormlike comb model whose main and side chains have different stiffness parameters. The molecular weight dependence of z-averaged mean-square radius of gyration (R-c(2))(0)) of the brush is determined and analyzed in terms of the wormlike cylinder model taking into account the end effects. The parameters characteristic of the rod brush in THF solution, such as the main chain stiffness parameter (lambda(-1)(M)), the molecular weight per unit contour length (M-L), and the excluded-volume strength (B), are determined and rationalized as a function of the contour length of the side rod. The polystyrene main chain stiffness of the rod brush remarkably increases by the densely located rod side chains to follow the scaling law of lambda(-1)(M) proportional to N-s(1). The backbone stiffness of the rod brush is higher than that of the flexible brush consisting of flexible polystyrene side chains with the corresponding contour length. A single rod brush of poly(VB-HIC-47) deposited on a mica surface is observed by the scanning force microscopy to reasonably demonstrate the cylindrical rod brushes.