The orbital angular momentum (OAM) of a vortex beam is measured and its performance is improved by gratings. Compared with other equipment or devices, this measurement method by gratings is simple and can make the cost of communication system reduced. The vortex beam is illuminated to an appropriate position of a period-gradually-changing grating or an annular grating. The light spot distribution in the diffraction pattern is observed and the incident vortex beam is measured. The experimental results show that the size and plus-minus of the topological charge of incident vortex beam can be determined according to the number and orientation of dark stripes in the light spot. Meanwhile, the phase correction or fan-out technique can be adopted to make the stripes more clearly visible in the diffraction results. Moreover, with these two techniques, the measured maximum number topological charges is increased to 30. This research provides a basis for the demultiplexing and generation of vortex beams in the OAM multiplexing communication.