The purpose of this study is to develop an economic and practical radiation sensor using photo diodes which is useful for estimating ET in greenhouse environment. An electronic circuit was then designed. Out put signal of photo diode is amplified with a LM318 transistor and is displayed in a LCD with value from zero to 255. The output signal was compared with that of a ready-made pyranometer. The behavior of the photo diode radiation sensor was evaluated by increasing number of photo diodes. The sensor became more reliable with increased number of photo diodes. Developed radiometer was tested with calibrated relation between pyranometer radiation and photo diode output. It was showed that the photo diodes radiometer would be applicable for the greenhouse environment.