Microscale gas gaps commonly exist in gas thermal conductance related microdevices, such as micro-hot-plate gas sensors and micro-Pirani vacuum gauges. In these devices, thermal conduction of the gas gaps is an important issue for their performance. Although simulations for size effect of the thermal conduction in microscale gas gaps have been carried out, experimental results are still rare. In this paper, four microhot plates that contain four gas gaps from 220 nm to 21 μm have been fabricated by a standard CMOS process and some additional post-CMOS processes. The thermal convection coefficient can be obtained as large as 1242 Wm2K-1 from the convection dominate 21- μ m gap. The effective thermal conductivity of 220-nm gap is as low as 1.2 × 10-3 Wm-1K-1. Both of them indicate that size effect of gaseous heat transport is significant in such microscale devices.