The micellization behaviors of ABA and BAB triblock copolymers in a solvent selective for block A are investigated by means of grand canonical Monte Carlo (GCMG) simulation. The basic micellar parameters such as the micellar size, its distribution, the micellar shape, and the internal structure of micelles are obtained and compared for both systems. The critical micelle concentrations (cmc) are quantitatively estimated from the osmotic pressure obtained by the GCMG simulation combined with the multiple histogram method. The results of simulation show that the chain architecture of the triblock copolymers shows large differences in their association behavior in a selective solvent, although both copolymers have the same composition and total chain length. The thermodynamic analysis confirms that an additional entropy loss due to the looping conformation of the middle block of BAB copolymer induces a higher cmc value and therefore reduces the capability to self-assemble into micelles. As a result, the BAB copolymer chain may have various chain topology such as dangling, loop, and bridge chains in micelles. From the multiple equilibrium model, it is possible to extract the entropy associated with packing the chains into a micelle for two different systems. The BAB copolymer yields less negative packing entropy due to less order of micelles by the presence of the dangling and bridge chains as compared with ABA copolymer, which results in micelles with larger sizes and a broader size distribution despite its high cmc value.