Beam diagnostic is an essential constituent of any accelerator. It is the organ of sense showing the properties and the behavior of the beam. One of the important parameter is beam energy that determines the application scope of accelerator. Monoenergitic accelerator is not available indeed beam particles in accelerators have energy distribution. In this thesis after introducing methods of energy measurement we focuse on the magnetic spectrometer and detection secondry particle. Methods the research is done for output beam of linear electron accelerator of Institute for research in fundamental science (IPM). For magnetic spectrometer a dipole magnet will be design by software manetic CST studio. Before designing dipole magnet and appropriate magnetic field validation software. Validation should be is done by magnetic H type and solenoid available in laboratory magnet. For production appropriate magnetic field a dipole magnet C type is designed. the maximum magnetic field maximum for electric current 11.5 Amper, is 0.387 Tesla. Then dimensions and materials needed to make the magnet spectrometer are presented. In the second method, the maximum energy of beam particles can be determined by detecting secondary particle. Secondary particls are bremsstrahlung photons that detected by photon detector. This method had been used for measuringof Yazd rhodotron accelerator. The feasibility of using this method for linac (IPM) is described in this thesis. Because of the secondary particles in this method have high energy, plastic detectors are needed for counting particles. In this detectors are Energetic particles are recorded in the Campton edge.