Using the bond-fluctuation model, we perform Monte Carlo simulations for polymer brushes composed of chains of two different chain lengths under good solvent condition. Profiles of monomer density and free end density, chain linear dimensions, and average monomer position along a chain are studied. Quantities measured in the simulations are derived from the analytic self-consistent field (SCF) theory and compared with the simulation data. The structural properties can be quite accurately described by the theory only when both the long and short chains are stretched. In general the predictions from the SCF theory are less satisfactory than in the monodispersed brush. The discrepancies are mainly attributed to the breakdown of the strong stretching condition of the longer chain units in the outer sublayer.