Diurnal variations of rainfall and upper wind over Bangladesh during the summer monsoon season were investigated through the data analysis of raingauge, 4-times-daily rawinsonde and pilot balloon observations. Over the northeastern part where the total monsoon rainfall is heavy a rainfall peak appears at 3 to 6 local time in the early morning. In the lower troposphere, the southeasterly accelerates in the evening, and the wind direction exhibits a clockwise change at night. This feature corresponds with the nocturnal jet, which can be a cause of the midnight-early morning rainfall peak in the northeastern part of Bangladesh through the increased wind blowing against the southern edge of the Meghalaya Mountains.