Abstract: We dicuss a fundamental theorem of phase contrast tomography,which plays a similar role as the Fourier slice theorem does for absorption-based CT.The theorem leads to develop a combined phase retrieval-reconstruction algorithm.With experimental results of In-line phase contrast CT from the Beijing Synchrotron Radiation Facility,we used the algorithm to reconstruct the three dimensional refractive distribution of the specimens,and compare the reconstruction quality for the conventional FBP algorithm.Some influencing factors of the algorithm has be dicussed.