It is recently recognized that sea-ice production in the polar sea-ice regions is controlled by the thin sea-ice area with thickness of less than 0.2 m. Spatial distribution of thin-ice area and its variability are important information for better understanding of the reduction of the sea-ice covered region due to the recent rapid climate change. We have been developing thin-ice thickness algorithm from satellite passive microwave data of the Advanced Microwave Scanning Radiometer–EOS (AMSR-E) and Special Sensor Microwave Imager (SSM/I). Although the microwave skin depth of bare sea ice is about several centimeters at most, microwave brightness temperatures correlate with the surface salinity (brine volume fraction), which is sensitive to thin-ice thickness. Thus, we made in situ observations by using heli-borne portable passive microwave radiometer which is the same sensor as the satellite AMSR-E, to improve or validate thin-ice thickness algorithm. This study estimates the relationship among the microwave brightness temperatures both from satellite and heli-borne portable sensors and in situ observations of sea-ice thickness, paying attention to the effect of sensor different spatial resolution (footprint) on the microwave characteristics. Under the Australian Antarctic Program (SIPEX; Sea Ice Physics and Ecosystem Experiment), we set portable microwave radiometer on the helicopter to observe surface brightness temperature (36 GHz-horizontal) of sea ice in late-winter 2007. Similar heli-borne observation was done during the sea-ice cruise in the Sea of Okhotsk in mid-winter 2009 using 36 GHz-vertical and -horizontal channel. These microwave data were compared with the satellite AMSR-E data and ice thickness estimated from moderate-resolution imaging spectroradiometer (MODIS) data, heli-bone radiation thermometer data, ship-borne downward-looking video data and ship-borne electromagnetic inductive sensor data for the observations from both regions. High-resolution heli-borne microwave brightness temperatures have good agreement with low-resolution satellite AMSR-E brightness temperatures. In the Okhotsk experiment where thin-ice region was dominant, polarization ratio of 36 GHz-vertical and -horizontal temperatures is found to be well correlated with ice thickness, supporting the validity of AMSR-E thin-ice algorithm, which is developed previously by our group. We further discuss the difference of thin-ice detection caused by the different spatial resolution and the validity of thin-ice algorithm in the marginal ice zone.