Abstract: A high brightness electron beam is required for free electron lasers. The design of an injector for a r.f. electron linear accelerator to provide a high brightness electron beam is presented. The injector consists of two standing wave cavity prebunchers and a fundamental frequency traveling wave tapered phase velocity buncher. The space charge effect and the beam loading effect are considered in the equation of beam dynamics. The beam pulse from the electron gun is of a Gaussian shape with the following parametes: the pulse width of electron beam T=4ns, the initial electron energy E₀ = 100keV, the peak current I = 5A. The calculation results with optimized parameters of the injector are: The bunch width and the energy spread in the bunch is less than 25ps and 200keV, respectively, the peak current is γ>400A, and the average normalized electron energy r is more than 4.