Synthesis of amorphous carbon fiber with atomic scale dispersed silicon was investigated. An organosilicon precursor (MSP), which was developed as heat resistant plastics in recent years, was used as a precursor. Temperature control during the melt-spinning was important in order to tailor precursor fibers having thin diameters. For the fiber curing, gamma-ray irradiation previous to pyrolysis in an O-2 atmosphere was available. The effect of irradiation on the MSP fiber was investigated with mass change, FT-IR and SEM observation after pyrolysis. During the irradiation, oxygen was introduced in the fiber in proportion to the total dose, but the irradiation curing formed microscopic defects on the fiber surfaces at the same time which limited the resultant fiber strength.