The variation of temperature and radius of typical micrometeoroids in the low thermosphere and mesosphere is calculated theoretically, and the formation of the spherule is explained consistently with this model. The fluffy particles floating in the stratosphere is also consistent with our calculation, since the particle of radius 10μ and initial velocity 20 km s−1 cannot be heated up to the sublimation temperature, even if it is not fluffy. Our calculation coincides exactly heights and velocities of meteor streams. The formation of the secondary particles from the evaporated vapour, is also calculated, and it is concluded that they are very few and small, compared with the primary particles.