Abstract A femtosecond pulse radiolysis with a time resolution of 210 fs has been developed by using a femtosecond electron beam and a femtosecond laser light. It has successfully opened the study of ultra-fast reactions or phenomena in materials—the first observation of the femtosecond formation process of the hydrated electron in water pulse radiolysis. The use of a photocathode femtosecond electron gun, which produces a near-relativistic 100 fs electron beam, has been approached to construct femtosecond megavolt electron diffraction. The dependencies of the emittance, bunch length, and energy spread on the radio-frequency (rf) and space-charge effects in the rf gun have been investigated.