Micellization of a diblock copolymer in dilute solution is studied by dissipative particle dynamics. The influence of the compatibility between blocks A and B and the interaction between the insoluble block and solvent on aggregation number P and micellar core radius Rc are examined. The micelle size distribution is obtained, and it is quite polydisperse. Different from the scaling theory for starlike micelles, the mean aggregation number based on weight average (P)w decreases with increasing soluble-block length NA and the power law relation can be obtained, (P)w ∼ NA-α Similarly, the micellar core radius declines with NA, following Rc ∼ AA -β with β = α/3. However, the exponent depends on the mutual compatibility between soluble and insoluble blocks. For the same composition, the incompatible diblocks form a smaller micelle and its aggregation number declines with a smaller exponent a. When NA ∼ NB, the micelles deviate significantly from the spherical shape and solvophilic blocks are observed to be entrapped in the solvophobic core for compatible diblocks.