Time-resolved small-angle X-ray scatterings have been measured by using synchrotron radiation for the isothermal crystallization process from the melt of the blends between the fully deuterated high-density polyethylene (DHDPE) and the hydrogeneous polyethylene with various degree of ethyl branching. The crystallization rate of the pure components estimated from the time dependence of the invariant is in the order of HDPE >> DHDPE > LLDPE(2) >> LLDPE(3), where HDPE is a high-density polyethylene (PE) without branching, LLDPE(3) is a linear low-density PE with ca. 17 ethyl branches/1000 C, and LLDPE(3) is a LLDPE with ca. 41 branchings. The similar crystallization rates of DHDPE and LLDPE(2) may be one of the most important origins governing the cocrystallization behavior between the D and H species. Pairs of DHDPE/HDPE or DHDPE/LLDPE(3) show basically the phase segregation phenomena between the D and H crystalline lamellae, possibly originating from the large difference in the crystallization rate between the D and H species. It has been found that the sample of DHDPE/LLDPE(2) blend, which shows the cocrystallization phenomenon, crystallizes faster than the original pure components; i.e., an acceleration effect has been observed for the blend. On the other hand, in the cases where the D/H pair shows the phase segregation, DHDPE/HDPE and DHDPE/LLDPE(3) blends, the crystallization rate has been found to be reduced largely after blended. The crystallization rate viewed from the crystalline trans band intensity, which was measured in the time-resolved FTIR experiments, has been found to be remarkably higher than that of the invariant evaluated from the SAXS measurement. This finding supports, for the first time, experimentally the following crystallization mechanism : the trans-zigzag chain segments are generated at first from the molten random coils and then these trans segments gather together to form small crystalline clusters, which grow into larger lamellae, as detected by the small-angle X-ray scattering measurement.