The self-assembly and the adsorption of a diblock copolymer, A(10)B(10), from a selective solvent have been simulated on a cubic lattice by a Monte Carlo method. The simulation uses the periodic boundary conditions in the X and Y directions and places two surfaces at the edges of the box in the Z direction. The A block is in a poor solvent, and B block, in a good solvent. Therefore, the former may absorb on the surface, and the latter may extend into the solution and form an external layer. The diblock copolymer may also self-assemble to form aggregates if the concentration in solution exceeds the critical micelle concentration. When an equilibrium is reached, there are three types of chains in the system : the free chains, the adsorbed chains, and the chains self-assembled in the aggregates. This report describes the simulation results of the equilibrium properties between the free chains and the aggregates in the existence of the adsorption. We find that the rate of change of the number of aggregates with concentration. We also find that above the cmc, the concentration of the free chains decreases linearly as the overall concentration of copolymer decreases, instead of keeping constant. The average number of chains and the polydispersity are calculated. The effect of the adsorption on the self-assembly of the diblock copolymer in solution is discussed.