Because of profound applications of two-dimensional molybdenum disulfide (MoS₂) and its heterostructures in electronics, its thermal stability has been spurred substantial interest. We employ a precision muffle furnace at a series of increasing temperatures up to 340 °C to study the oxidation behavior of continuous MoS₂ films by either directly growing mono- and few-layer MoS₂ on SiO₂/Si substrate, or by mechanically transferring monolayer MoS₂ or hexagonal boron nitride (h-BN) onto monolayer MoS₂ substrate. Results show that monolayer MoS₂ can withstand high temperature at 340 °C with less oxidation while the few-layer MoS₂ films are completely oxidized just at 280 °C, resulting from the growth-induced tensile strain in few-layer MoS₂. When the tensile strain of films is released by transfer method, the stacked few-layer MoS₂ films exhibit superior thermal stability and typical layer-by-layer oxidation behavior at similarly high temperature. Counterintuitively, for the MoS₂/h-BN heterostructure, the h-BN film itself stacked on top is not damaged and forms many bubbles at 340 °C, whereas the underlying monolayer MoS₂ film is oxidized completely. By comprehensively using various experimental characterization and molecular dynamics calculations, such anomalous oxidation behavior of MoS₂/h-BN heterostructure is mainly due to the increased tensile strain in MoS₂ film at elevated temperature.