Starting from June 1986 a single cavity RF gun with thermionic cathode(LaB6) was studied and constructed at the Institute of High Eneergy Physics, Academia Sinica, Beijing Free Electron Laser Project. In the experiments the electron back-bombardment effect and another phenomenon connected with cathode temperature were obseerved. After carefuly choosing the opeeration parameters, this RF gun injector could stably provide clectron beams suitable for doing preliminary FEL experiments. Based on the theoretical analysis a computer code(including space charge forces) was written to simulate the dynamics of electrons in this injector. Since May 1986, at LAL, Orsay, France, the photocathode RF gun was investigated theoretically and new design criteria were established. Based on these criteria a two decoupled cavities RF gun, using a laser triggered photocathode, has been designed for the "LAL/Orsay RF gun project"(CANDELA), and model cavities have been fabricated. Based on the model cavity measurement results, the final cavities have been fabricated and measured. Aiming at reducing further the emittance of electron bunches coming out of the second cavity, Linear and nonlinear emittance compensating techniques have been proposed. As a new kind of electron injector suitable for FEL and future e+e- linear colliders, a travelling wave RF gun has been investigated theoretically and compared with the standing wave RF gun. Some preliminary numeerical calculations have been made also. Stimulated by the experimental results on the model cavities, several useful analytical formulae have been derived and verified by the experimental results: (1) Analytical formulae for the coupling coefficient β of a cavity-waveguide coupling system (2) Analytical formulae for the resonant frequency changes due to opening apertures on cavity walls (3) Analytical formulae for the coupling coefficient k(or group velocity vg) of forward and backward travelling wave linear accelerators