The crystallization behaviour of an epsilon-caprolactone-block-butadiene diblock copolymer was observed by small-angle X-ray scattering employing synchrotron radiation. The processes of primary and secondary crystallizations were separately analysed by procedures usually used for homopolymer crystallization, and were compared with the case of a poly(epsilon-caprolactone) homopolymer (PCL). The Avrami analysis at the primary crystallization showed an exponent n ranging from 2 to 3. This value is comparable to n evaluated for PCL and also n widely reported for crystalline homopolymers, indicating that the crystallization of the PCL block drives the primary stage without the influence of the existing microphase structure of the block copolymer. In the secondary crystallization, on the other hand, the rate was significantly retarded when a microphase structure existed in the system.