A virtual photobioreactor for mass-culturing microalgae was developed. This is a computer model system combining a numerical simulation code for two-phase turbulent flow of bubbles and continuous medium and a photosynthesis model that can express the flashing light effect. The flashing light effect should be considered because turbulent flow in the reactor gives microalgae a chance to come close to the irradiated surface in the opaque medium at irregular frequency and this intermittent illumination enhances photosynthesis of the algae. The two-phase flow model output the time history of light pass along light ray between the irradiated wall surface of the reactor and the individual algal cell, which was passively moved by turbulent flow. When the history of light intensity, which was calculated from that of the light pass and the Beer-Lambert law, experienced by the cell was given, the photosynthesis model output the amount of O-2 emitted from the cell in every small time interval. Finally, the harvest of the alga was estimated from the amount of the O-2. As a result, the present model system successfully predicted the algal concentration optimal for the largest O-2 emission at the given light intensity and simulated the growth curve of Chaetoceros gracili. (C) 2010 Elsevier Ltd. All rights reserved.