Field study was conducted to know the effect of thermal environment on different phenostages, grain yield andyield attributes of fababean (Vicia fabaL.) during rabi season of 2013-14 and 2014-15 at ICAR Research Complex for Eastern Region, Patna, (Bihar). Fababean was sown four dates (Nov. 1, 15 & 30, Dec. 15) with four varieties(Local, Vikranta, Gaurav and Suraksha).Yield of fababean varieties was strongly responsive to the thermal units/growing degree days (GDD) accumulated during emergence to maturity. Results revealed that duration of phenostages and thermal units varied with the dates of sowing. November 01 sown crop produced significantly higher number of seeds pod-1 (3.21), higher seed yield (3.13 t ha-1) and biological yield (6.30 t ha-1) compared to other sowing dates. Among varieties, Gaurav exhibited significantly higher total dry matter production, heat use efficiency (2.32 kg ha-1°C day), more number of seeds pod-1 (3.03) and higher seed yield (3.47t ha-1) followed by Suraksha, Vikranta and Local.