The micellization behaviors of two types of ABA and BAB triblock copolymers with different block compositions in a solvent selective for block A are investigated by using the grand canonical Monte Carlo simulation combined with the multiple histogram method. The block composition and sequence are found to significantly control the micelle formation and the association behavior for both types of triblock copolymers. The thermodynamic analysis shows that DeltaH(0) and DeltaS(0) decreases as the length of insoluble block B decreases, indicating that the driving force toward micellization decreases with decreasing the B block length for both types of copolymers. On the other hand, BAB copolymers show less negative DeltaH(0) and more negative DeltaS(0) than ABA at all the compositions except at f(B)=0.20, where the reduced capability of BAB copolymer to form micelle leads to increase the amount of dangling chains in the micelle, yielding less negative DeltaS(0). The basic micellar parameters such as the micellar size, the micellar shape, and the distribution of segments in a micelle are examined as a function of block composition and sequence. It is evident that the micelle formation of triblock copolymer follows the closed association mechanism, irrespective of the block composition and sequence.