Photothermal Quotient as combined effect of temperature and solar radiation were studied as determinant factor for spring wheat grain yield. The data obtained at anthesis and maturity for grain m-2, grain weight and grain yield were examined in experiments involving three wheat genotypes under five different environmental conditions (E) provided in the form of planting windows (PW’s) during 2008-09 & 2009-10 at National Agricultural Research Center (NARC), Islamabad. The mean temperature at anthesis (T1) and maturity (T2) was calculated by averaging all temperature from germination till anthesis and maturity, respectively. Similarly, solar radiation at anthesis (SR1) and maturity (SR2) was calculated- with the Angstrom formula while Photothermal quotient (PTQ) was calculated at anthesis (PTQ1) and maturity (PTQ2). The data obtained was subjected to STATISTICA 8 software and scatter plot regression model was developed- at 95% confidence interval with crop data and climate variables (T1, T2, SR1, SR2, PTQ1 and PTQ2). The b (regression coefficient) recorded were 623.73 (GM vs T1), 0.0037 (GW vs T1), -5.97 (Y vs T1), -0.028 (GW vs T2), -1356.28 (GM vs T2), -57.86 (Y vs T2), 29.53 (GM vs SR1), 0.0005 (GW vs SR1), 3.95 (Y vs SR1), 19.90 (GM vs SR2), 0.0003 (GW vs SR2), 2.57 (Y vs SR2) 314.20 (GM vs PTQ1), 0.0049 (GW vs PTQ1), 49.88 (Y vs PTQ1), 0.0037 (GW vs PTQ2), 31.87 ( Y vs PTQ2), 237.40 (GM vs PTQ2) while, b for E and PW with yield was -872.51 and -309.75, respectively. Direct relationship between PTQ and yield parameters confirmed that it determined crop yield and its management for variable environmental conditions need to be opted by adopting suitable sowing time.