Prototype bioreactor using internal lighting mechanism was manufactured for biological CO2/O2 conversion system. Microalgae culturing experiments were carried out on Chlorella ellipsoidea C-27 to estimate the effect of light intensity on photosynthetic activity in the reactor. The light energy source used was an Xe(xenon) lamp, from which light is transferred through optical fibers and diffused from the surface of rods made of acrylic resin. Total surface area of the rods was 0.12m2.Tests under the continuous lighting (600W) condition indicated a highest specific growth rate of 8.54 (h-1), a highest cell density of 1.3 × 108 (cells/ml), and a maximum overall photosynthetic rate (CO2 absorption rate) of 171.84 (mg-CO2 absorption/Ir-medium/day). In contrast, the maximum CO2 absorption per chlorophyll content (mg-CO2 absorption/mg-chl/h) was reached under the alternating lighting condition. Comparison on the photosynthetic rate under several conditions suggested that the most suitable condition for the synthetic rate did not coincide with the best condition for the photosynthesis of the intact cell. Relationships were also investigated, under various growing conditions, between the ratios of chlorophyll a and chlorophyll b versus the CO2 absorption per chrolophyll content. Results of the studies suggest the importance of measuring chrolophyll a and b for monitoring photosynthetic abilities of growing cells in a vessel.