Abstract: Orbital angular momentum (OAM) of light beams attracted great attention recently due to its importance in theory and application. Most works on OAM of light beams were concenterated on coherent beams, and few efforts were made on partially (spatial) coherent beams (PCB). The OAM of PCB was deduced from their Wigner distribution function, and the OAM spectra of PCB were analysed. The Mercer expansion method was used to obtain the OAM spectra of PCB. For given PCBs, their OAM spectra were carried out. The dependence of OAM spectra on the partial coherence was also discussed. We found that the OAM of the PCBs takes the same form as that of coherent beams, the OAM spectrum of PCBs disperses more seriously with the worse coherence, and the OAM spectrum of PCBs with zero total OAM is more vulnerable to bad coherence than that of the PCBs with non-zero total OAM.