CaO-SiO2 gels for bioactive organic-inorganic composites were prepared from tetraethoxysilane (TEOS) and calcium nitrate tetrahydrate (Ca(NO3)2⋅4H2O) by a sol-gel method with the addition of the various amount of polyethylene glycol (PEG). The effects of PEG on morphology and bioactivity of the gel were investigated. The samples with the starting compositions of Ca(NO3)2 : TEOS = 30 : 70 in molar ratiowere prepared with or without coexistence of PEG. All the examined gels were obtained as a crack-free bulk bodies after gelation and aging. FT-IR spectra of the prepared gels showed that the gels have similar contents of silanol groups regardless of the amount of PEG. SEM observation showed that the gel consists of aggregates of 2-3 μm spherical particles with a diameter of 2-3 μm when it was prepared with the addition of PEG at a molar ratio of PEG : TEOS = 0.002 : 1, while it consists of aggregates of spherical particles with a diameter of 50 nm at a molar ratio of PEG : TEOS = 0.004 or 0.007 : 1. The size of the CaO-SiO2 powder can be decreased with increasing the amount of the added PEG. All the samples, regardless of the amount of the added PEG, formed apatite on their surfaces in a simulated body fluid (SBF) within 1 d. These results suggest that bioactive fine spherical particles with various size can be obtained through sol-gel processing under co-existence of appropriate amounts of PEG.