High-temperature-resolution small-angle X-ray scattering (SAXS) experiments were performed to elucidate the order-disorder transition at temperature T (T-ODT) and the crossover from the disordered state characterized by the Brazovskii-type non-mean-field theory to the disordered state characterized by the Leibler mean-field theory at T (T-MF) for low molecular weight polystyrene-block-polyisoprene copolymers having about equal block molecular weights. Across T-ODT, the inverse peak scattered intensity I-m(-1) and the peak width sigma(q) clearly show a very sharp discontinuity, but the characteristic length D of the concentration fluctuations shows almost no change. The second-order scattering maximum existing below T-ODT definitely disappears above T-ODT, indicating that the spatial concentration fluctuation profile changes from a square-wave-type to a sinusoidal-wave-type profile upon increasing T through T-ODT. The crossover temperature T-MF is clearly identified from the changes in the temperature dependencies of I-m(-1), sigma(q)(2), and D with T : partial derivative I-m(-1)/partial derivative(1/T), partial derivative sigma(q)(2)/partial derivative(1/T), and partial derivative D/partial derivative(1/T) all show a discontinuity at the same temperature T-MF. The temperature dependence D at T > T-MF is explained by that of the radius of gyration R(g0) of the unperturbed block copolymer chains, while that at T < T-MF is much greater than that of R(g0).