Aimed at investigating the relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel, the fully austenitized steel was cooled to a temperature above or below the start temperature of martensite transformation (M s o , 406 °C), which occurs upon continuous cooling from the austenitizing temperature to room temperature, and held for different times (10 min, 25 min, and 45 min) followed by cooling to room temperature. The concept of primary and secondary martensite is introduced to indicate that two different, sequential, martensites formed in this processing. When cooled to 650 °C or 450 °C for different times, the austenite stability decreased with prolonged holding time, and during the subsequent cooling the primary martensite start temperature ( M s ) was raised to a value higher than M s o . In contrast, M s was reduced to a level lower than M s o when cooled to 420 °C or 410 °C for 45 min. On the other hand, as the steel was isothermally quenched at temperatures below M s o , during the subsequent cooling a secondary martensite reaction could be observed, and the secondary martensite start temperature (M s ′ ) decreased with holding time. Retained austenite in the form of thin films along martensite lath boundaries was obtained when cooled to 380 °C or below for different times.