It has been proved that exact solution of k-center problem is NP hard.Approximation to the problem was found to be also NP hard when the error is small. The error level in the approximation algorithm is measured as an approximation factor, which is defined as the ratio between the approximation and the optimum. It's proved that thek-center problem approximation is NP hard when approximation factor is less than 1.822 (dimension = 2) or 2 (dimension > 2).