Comparison and discussion of several boundary conditions in lattice Boltzmann method are presented. To achieve improved accuracy, a new method based on the idea of standard bounce-back scheme is proposed. Numerical simulations for two-dimensional Poiseuille flow and unsteady Couette flow are carried out using d2q9 model, and the results are identical to the analytical solutions. The convergence for different boundary treatments is discussed by changing single relaxation time τ and Re number. The results show that LBM can recover Navier-Stokes equation at low Mach number. It is also indicated that the errors for large τ are much bigger than those for τ close to 1.