Abstarct: This study was aimed to investigate the radiation use efficiency and light extinction coefficient for cowpea plants (cultivar Bastami) at 4 sowing dates 9 Jun, 26 Jun, 13 Jul and 9 August and planting space within row 7, 10.13 and 16 cm during the 2010-2011 year on farms of College of Agriculture, Shahrood University of Technology which is located in the Bastam city. The experimental design was factorial baxsed on randomized complete block design with three replications. Planting dates and densities were chosen just for creating variety of conditions of solar radiation absorption and then, quantifying radiation use efficiency and extinction coefficient. The results showed that delaying in planting date decreased linearly the growing degree days. So that, the longest period of growth was found for 9 June. The growing degree days achieved during this planting date was 1563. Analysis of variance showed that all attributes except the extinction coefficient and radiation use efficiency were significantly different at the 1% probability for tested planting dates and planting densities. The mean comparison results showed that the highest value of leaf area index, total dry matter production, grain yield, the intercepted light and ratio of intercepted radiation at the time of flowering was found for 7 cm row spacing and planting date of 9 June. Extinction coefficient was not affected by plant density sowing date, and the mean value was 0.622. Radiation use efficiency was ranged from 0.785 to 0.844 Mj.m-2. Temperature is one of the most important factors in light use efficiency. In this study, the mean temperature in the growing period varied between 18.9 and 22.5 5/22 °C.