Abstract: Production of biodiesel from microalgae is a promising step toward sustainability. The superiority of microalgae to their counterpart sources of biomass lies in many favorable characteristics of microalgae such as high biodiesel productivity, low land use and carbon dioxide biofixation. Sunlight as an effective parameter in growth and cultivation of various microalgae species is an aspect of concern in design and operation of photobioreactors. In this communication, we present a simple measuring system which can provide the effect of sunlight irradiance in photobioreactors qualitatively. The system is very low cost and can be connected to an ordinary personal computer to monitor sunlight intensity to a photobioreactor. Based on a set of experiments a calibration curve for the measuring system is also given. The proposed system can contribute to optimal growth of microalgae inside large scale photobioreactors as well as to lab scale research projects.