This paper reports thermal stability of particulate media, which includes Co-Fe oxide, CrO_2,thick and thin MP tapes. Measuring the time decay of magnetization at room temperature, fluctuation fields were obtained as a function of the reversely applied field. It was clarified that the fluctuation field has a constant and minimum value when the reverse field is equal to coercivity. Based on this result, fluctuation fields for the four particulate tapes were measured at several environmental temperature, ranging from -75℃ to+100℃. It was clarified that the fluctuation field normalized by remanence coercivity increases as the environmental temperature increases for all tapes, indicating that becomes a good measure of thermal stability. Activation volumes were also reduced as a function of temperature.