Abstract
This paper is concerned with using two spectral data to determine the piecewise constant density function of a string equation. We will also prove that if the Dirichlet-Dirichlet spectrum and the Neumann-Dirichlet spectrum of a string equation are the same as these of a homogenous string equation with the density function which equals to 1 on [0,1], then the density function is equal to 1 almost everywhere on [0,1].