Algae-organisms are unicellular or simple multicellular body plant that is able to manufacture their own food material by photosynthesis. Algae can be cultivated under certain conditions of temperature, light and sufficient nutrients to produce biodiesel. Many researchers worked to increase the efficiency of the production rate in both indoor and outdoor cultivating systems and by designing special photobioreactors (PBRs) to increase the rate of photosynthesis. Light conditions affect directly the growing and photosynthesis of microalgae.(Duration and intensity). Microalgae needs a light/dark regime for productive photosynthesis, it needs light for a photochemical phase to produce (ATP, NADPH) and also needs dark for biochemical phase synthesize essential molecules for growth. This article critically reviews research under taken to date to study the effect of light on the microalgae growth.