The effects of C doping and Tb co-doping on the enhancement of visible luminescence from silicon-rich silicon oxide (SRSO), which consists of Si nanoclusters embedded inside an SiO2 matrix, are investigated. C-doped SR.SO (SRSO : C) and Tb co-doped films were fabricated by an electron cyclotron resonance-plasma enhanced chemical vapor deposition method with concurrent sputtering of Tb target by using SiH4, O-2, and CH4 source gases followed by a high temperature anneal. Intense blue-white visible luminescence, visible to the naked eye under daylight conditions, was observed from SRSO : C film with a nearly equal amount of C and excess Si, after an anneal at 950 C and furthermore tuned by controlling the C to excess Si ratio, the C content, and the anneal temperature. The film co-doped with Tb also shows strong Tb3+ photoluminescence which is enhanced by the content of C. The 543 nm Tb3+ lifetimes were in the range of 0.5 - 1.2 msec, comparable to those from Tb-doped silica. Based on the results, we conclude that exciton recombination in C-incorporated Si nanoclusters is responsible for visible luminescence of SRSO : C and can efficiently excite incorporated Tb3+ ions.