We consider the problem of determining a pair of functions (u, f ) satisfying the heat equation u t À Áu ¼ 'ðtÞfðx; yÞ, where ðx; yÞ 2 ¼ ð0; 1ÞÂð0; 1Þ and the function ' is given. The problem is ill-posed. Under a slight condition on ', we show that the solution is determined uniquely from some boundary data and the initial temperature. Using the interpolation method and the truncated Fourier series, we construct a regularized solution of the source term f from non-smooth data. The error estimate and numerical experiments are given.