We investigated the feasibility of proton therapy dosimetry by using bare silica glass optical fibers. A silica glass fiber, with 400μm core diameter, was placed in proton radiation fields generated by a proton therapy cyclotron and simultaneously luminescence spectroscopy was performed to analyze the emission spectrum of the fiber tip. In order to measure the radiation absorbed dose at various depths in tissue-mimicking media, the fiber tip was embedded in a plastic slab and additional slabs of phantom were added sequentially. The spectrum of the irradiated fiber over the 400-700 nm sensitivity range of the spectrometer shows two distinct peaks at 460 and 650 nm, whose spectral shape is different from that of erenkov radiation. We found that the emission peak at 650 nm shows correlation with the radiation absorbed dose measured by a standard ion chamber device indicating the feasibility of proton dose measurement by using a bare silica fiber.