Plantation of trees in Nelder wheel design provides varied microclimatic conditions especially in terms of solar radiation penetration for understory crop in agroforestry system. The present investigation attempts to study the diurnal radiation over wheat as an understory crop below 8 year old Eucaluptus tereticornis planted in a Nelder wheel design planted in fifteen spokes of trees numbered serially starting from north direction as 00 . Wheat cv. PBW-226 was sown on 21st November, 1996 as an under story crop. Studies have been made on the diurnal variation of net radiation attributable to orientation of tree rows of the trees at four important phonological stage of the crop i.e. tillering, flag leaf emergence, flowering and maturing stage of the crop and compared with the control (sole crop) viz T1 (312O-72O), T2 (72O-192O) and T3 (192O- 12O) in the Nelder wheel. Regression equations between net radiation in control and different treatments below trees were also developed.