A micrometeorological method was used to measure the diurnal exchange of carbon dioxide and methane over Sundarbans mangrove forest at Lothian island under lapse and inversion micrometeorological conditions during 18th - 19th May, 1999. Concentration of CO2 and CH4 in the air were found to be 587.9±51.5 and 0.903±0.318 mg/m3 and both vertical and lateral transport could cause their variation in the atmosphere. This island was a net source of both CO2 and CH4 during the study period and their overall diurnal emission rates estimated to be 113.06 and 1.51x106 mg/d, respectively. Micrometeorological conditions of the atmosphere played an important role for the exchange of carbon dioxide and methane between atmosphere and biosphere.