It is ordinarily considered that hydrocarbons production from kerogen in nature is carried out in reduced environments. Heating experiments on living pollen and spores as a simulation of organic maturation have been generally conducted in open system under the air and nitrogen gas. In order to investigate the influence of air and nitrogen gas on thermal alteration of pollen, living pollen grains of Pinus thunbergii were heated in open system to 290℃ under the air and to 453℃ under the nitrogen gas. The temperature to begin abrupt weight loss, contraction and color darkening (decrease in statistical Thermal Alteration Index) of pollen grains heated under the air is lower than that heated under the nitrogen gas. The pollen grains heated at the temperature higher than 180℃ under the air might be oxidized by atmospheric oxygen, because atomic O/C ratio of those grains are higher than those heated under the nitrogen gas. Our results support that inert gas including nitrogen gas is better than the air as the atmosphere of heating experiments for simulation of organic maturation.