The domain structure and crystalline morphology of AB and ABA type block copolymers of ethylene oxide (EO) and isoprene (Ip) were investigated by using two kinds of casting solvents, non-selective (benzene) and selective (ethyl benzene) solvents. The domain structures of both types of block copolymers are identical, as in the case of amorphous block copolymers, depending mostly on the fractional compositions of each block segment but hardly on the sequence arrangements. When benzene is used as a cast solvent, the change of domain structure with increase of Ip fraction follows, in general, a role established for amorphous block copolymers, except for the fact that the EO segment is crystallizable so that the above role is considerably modified, especially when the EO fraction is dominant, to form spherulitic crystalline texture. The domain structure of mixed system of block copolymer with homo-PEO, cast from benzene solution, also follows the above role, simply depending on fractional composition as a whole, of each component, unless the molecular weight of the homopolymer is much larger than that of corresponding block segment. On the other hand, when ethyl benzene is used as a cast solvent, the domain structure is quite different from that cast from benzene solution, giving single crystal-like texture of EO segments on which Ip segments are segregated to form Ip layer even for a copolymer having an EO fraction as small as 20%. The above contrast of domain formation mechanism between the two kinds of casting solvents is interpreted in terms of an interrelation of two binodal surfaces, critical concentration of crystallization of EO segments and critical concentration of micelle formation of EO and Ip segments, in the phase diagram of the system at a given temperature.