The study investigates the thickness-dependent phase stability of ultrathin 2M WS₂, a type-II transition metal dichalcogenide. Through systematic experiments and theoretical simulations, the researchers find that ultrathin 2M WS₂ exhibits significantly higher intrinsic thermal stability compared to bulk samples. The 2M-to-2H phase transition temperature increases from 120 °C to 210 °C as the thickness of WS₂ decreases from bulk to bilayer. Monolayered IT' WS₂ can withstand temperatures up to 350 °C in air and 450 °C in an argon atmosphere before being oxidized or transforming to the 1H phase. The enhanced stability is attributed to stiffer intralayer bonds, increased thermal conductivity, and a higher average barrier per layer during the layer-by-layer phase transition process. The findings suggest that the high intrinsic phase stability of ultrathin 2M WS₂ can expand its practical applications in various fields, including superconductivity, electronics, and energy conversion and storage.The study investigates the thickness-dependent phase stability of ultrathin 2M WS₂, a type-II transition metal dichalcogenide. Through systematic experiments and theoretical simulations, the researchers find that ultrathin 2M WS₂ exhibits significantly higher intrinsic thermal stability compared to bulk samples. The 2M-to-2H phase transition temperature increases from 120 °C to 210 °C as the thickness of WS₂ decreases from bulk to bilayer. Monolayered IT' WS₂ can withstand temperatures up to 350 °C in air and 450 °C in an argon atmosphere before being oxidized or transforming to the 1H phase. The enhanced stability is attributed to stiffer intralayer bonds, increased thermal conductivity, and a higher average barrier per layer during the layer-by-layer phase transition process. The findings suggest that the high intrinsic phase stability of ultrathin 2M WS₂ can expand its practical applications in various fields, including superconductivity, electronics, and energy conversion and storage.