The multiple input multiple output (MIMO) technique enables the application of spatial multiplexing as a way of increasing the spectrum efficiency. However, only a few antenna branches are allowed at a simple mobile terminal (MT) and the MIMO effect is limited for those MTs. Multiuser (MU)-MIMO techniques were proposed to support such simple terminals to overcome such problems. Among them, the block diagonalization algorithm is well known as a practical linear precoding technique for downlink in the MU-MIMO systems. Because the number of users supported by the access point is limited and the improvement in the spectrum efficiency greatly depends on the combination of spatially multiplexed user, a simple transmission performance estimation method is needed. In this paper, we propose a new capacity estimation method for MU-MIMO systems. The proposed method estimates the achievable bit rate in MU-MIMO while significantly reducing the calculation load compared to that for direct estimation in the block diagonalization algorithm. Theoretical analysis and the simulation results confirm the validity of the proposed method.