Sorghum is one of the major food crops it was consumed as flour, grains and fodder for animals. A field experiment was carried out at Tamil Nadu Agricultural University, Coimbatore to optimize the sowing window for Rabi sorghum 2019 based on thermal indices. The treatment comprises of three sowing dates viz., First sowing (D₁ – 15^th September), Second sowing (D₂ – 30^th September) and Third sowing (D₃ – 22^nd October). Growing degree days, Helio thermal units, Photo thermal index, Heat use efficiency, Helio thermal use efficiency was calculated for different phenophases of sorghum viz., germination, vegetative, flowering, physiological maturity. The results indicated that first sowing (first fortnight of September) accumulated growing degree days, Helio thermal units, Heat use efficiency, Helio thermal use efficiency and also higher grain yield compared to other sowing dates.