With a steady temperature increase under high vacuum (HV) in an environmental scanning electronic microscope, we observed charge-free characterization and fine secondary electron (SE) images in focus for insulating ceramics (alumina (Al2O3), aluminum nitride (AlN), pure magnesium silicate (Mg2SiO4)). The sample current I(sc) increased from -8.18 x 10(-13) to 2.76 x 10(-7)A for Al2O3 and -9.28 x 10(-12) to 2.77 x 10(-6)A for AlN with the temperature increased from 298 to 633K. The surface conductance sigma increased from 5.6 x 10(-13) to 5.0 x 10(-11)/Omega for Al2O3 and 1.1 x 10(-12) to 1.0 x 10(-7)/Omega for AlN with the temperature increased from 363 to 593K. The SE image contrast obtained via heating approach in high vacuum with an Everhart-Thornley SE-detector was better than that via conventional approach of electron-ion neutralization in low vacuum (LV) with a gaseous SE-detector. The differences of compensation temperatures for charge effects indicate dielectric and thermal properties, and band structures of insulators. The charge compensation mechanisms of heating approach mainly relate to accelerated release of trapped electrons on insulating surface and to increase of electron emission yield by heating.