Radiation damage of amorphous Si (a- Si) under 17 MeV- proton bombardment has been studied, measuring particle- induced conductivity(PIC) and photoconductivity(PC). The in- situ measurements have been conducted to focus on the structural flexibility and the metastable nature, inherent in the amorphous structure. While the PC (with weak illumination) has a fast response and reversible nature, the PIC has a long time- constant ∼60 s, followed by a persistent conductivity for more than ∼103 s. Both the PIC and the PC remain fairly stable against further irradiation, comparing to crystalline Si (c-Si), but they decay in the higher fluence region. A large part of the decayed PC recovers after annealing at 450 K. The results of a- Si are compared with those of c- Si. It is suggested that, in the lower fluence region, a-Si is more resistant against proton irradiation than c- Si, relaxing the atomic displacements, and the excited conductivities gradually decay with accumulation of the dangling bonds.