Investigations were conducted to study the thermal energy utilization by sunflower during spring 2014 and 2015 at Punjab Agricultural University, Ludhiana, Punjab, India. Experiments were laid out with three sowing dates (January 20, February 10 and March 2), two intra rowspacing (30 and 24 cm) and four nitrogen doses (0, 45, 60 and 75 kg ha-1). The results indicated that early sowing and application ofnitrogen significantly increased number of days to attain physiological maturity. The thermal units required to attain various phenological stages also increased progressively as the sowing was delayed. However, effect of sowing date was not conspicuous onaccumulated growing degree days (AGDD) to attain physiological maturity. W idely sown crop required slightly more AGDD than closely sown for attaining physiological maturity. Application of nitrogen also increased the AGDD. Heat use efficiency followed the descending order with each successive delay in sowing.Seed filling duration had highly significant (P<0.01) positive correlation and regression with seed yield, indicating that with each unit increase in seed filling duration, seed yield increased by 91.3 units.