Abstract: Water activity (a_w) is the key factor defining low-moisture foods (LMFs, a_{w,25 ℃}<0.85), and also determines the heat resistance of microorganisms in LMFs. However, a_w of food is a temperature-dependent factor, and is highly influenced by food components and texture. When temperature rises, a_w has shown different trends of changes. Water activity at isothermal temperature (a_w,T) reflects the real-time a_w of foods during high-temperature pasteurization. Accurate measurement and calculation of a_w,T enable scholars to quantify the heat resistance parameters of microorganisms in LMFs, and to design, verify, and improve associated thermal pasteurization processes. However, commercial a_w meters can only conduct the measurement at low temperature range (20~50 ℃). Little is known on the a_w in the high temperature range (>55 ℃). The microbial safety of LMFs and its microbial heat resistance parameters are first introduced in this article. Then the definition, detection methods, model predictions, influencing factors of a_w,T and the effects of a_w,T on microbial heat resistance of LMFs are summarized. Recognition and measurement of a_w,T are beneficial for food industries to reduce the degree of LMFs microbial contamination and ensure consumers food safety.