We consider the solutions of block Toeplitz systems with Toeplitz blocks by the preconditioned conjugate gradient (PCG) method. Here the block Toeplitz matrices are generated by nonnegative functions f(x, y). We use band Toeplitz matrices as preconditioners. The generating functions g(x, y) of the preconditioners are trigonometric polynomials of fixed degree and are determined by minimizing ∥(f - g)/f ∥∞. We prove that the condition number of the preconditioned system is O(1). An a priori bound on the number of iterations for convergence is obtained.