Blends of poly[(R)-3-hydroxybutyrate] (PHB) with poly(L-lactide acid) (PLLA) were prepared to study the crystallization behavior of PHB in the blends. To investigate the effect of molar mass of PLLA on the crystallization of PHB, two types of PLLA with high and low molecular weight, HPLLA and LPLLA, were used. It was found that the crystallization behavior of PHB in PHB/HPLLA and PHB/LPLLA blends is essentially the same for thicker films, e.g., 1 mu m thick films. The crystallization behavior of PHB within PHB/HPLLA and PHB/LPLLA blends in ultrathin films is, however, quite different. For PHB/LPLLA thin films, it was found that crystallization degree of PHB decreases first slightly with increasing LPLLA content, and then the crystallization stops when the weight ratio of PHB (w(PHB)) decreases to 30 wt %. On the other hand, for the PHB/HPLLA thin films, the crystallization of PHB is inhibited even at extremely high weight ratio of PHB (W-PHB <= 78 wt %). It was further found that thermal annealing does not enhance crystallization ability of PHB in PHB/HPLLA thin films with W-PHB <= 78 wt %. Infrared reflection-absorption spectroscopy (IR-RAS) suggested the existence of an intermediate state of PHB. The transition from the intermediate state to crystalline phase is significantly slowed down in the thin film of PHB/HPLLA blend with W-PHB <= 78 wt %.