Microclimatic alteration produced by mulching with different coloured plastic sheets, inside and outside the polyhouse and its effect on yield of tomato was studied during 2012-13 at Birsa Agricultural University, Ranchi. The mean weekly minimum and maximum air temperature during last week of December to 1st week of March were found to be higher by 2 to 9 0C inside the polyhouse than open field. Relative humidity was always higher in the open field during January to February by 2 to 7 % but it was higher inside the polyhouse in the months of March to May by 4 % at 7.00 AM. Almost a similar trend at 2.00 PM was also observed but during March to April relative humidity was higher by 10 % in polyhouse condition. The maximum available light intensity inside the polyhouse was about 30 to 40 % lower than that of the open field irrespective of growth stages. Average soil temperature was found to be higher by 2 to 5 0C under open field condition than inside the polyhouse. Leaf temperature of tomato grown under polyhouse was always lower than the open field condition. These microclimatic conditions inside the polyhouse favoured the performance of tomato and fruit yield obtain from the polyhouse was 65 t ha-1 against 33 t ha-1 from the open field