The experiment was conducted at Plant Pathology Field, Bangladesh Agricultural Research Institute (BARI), Gazipur-1701; Regional Agricultural Research Station (RARS), Jamalpur and Regional Agricultural Research Station, Rangpur during the cropping season 2014-17. Efficacy of environmental factors on alternaria blight disease of mustard was studied with six different dates of sowing. The highest disease score 5.1 was recorded at Gazipur in December 10 sowing followed by 5.0 at Rangpur in December 01 and December 10 sowing. The lowest disease score 2.2 was recorded in November 01 sowing at Jamalpur. The highest seed yield 1.55 ton/ha was observed at Rangpur and the lowest seed yield 0.54 ton/ha was found in December 10 sowing at Gazipur.The average air temperature was 23.6º C, 22.2º C and 23.1º C at Gazipur, Jamalpur and Rangpur locations during November and it was below 20º C in all three locations during December and January and then rises at February and later.