Based on the in situ XBT and other data sets, by analyzing the seasonal cycle of the mixed layer depth (MLD) and using the conservative potential vorticity (PV) as a tool, a clear description of the formation process of the North Pacific Subtropical Mode Water (NPSTMW) is presented for explaining the well known ’Stommel Demon’. The forming of NPSTMW reflects well the ventilation process of the isotherms of the permanent thermocline. The formation process can be divided into the ’ventilation’ phase and the ’formation’ phase. In the first phase (October-March), with large heat losses at the sea surface from October, the mixed layer deepens and correspondingly, the water mass with low PV emerges and sinks. After continual cooling from October to March, the mixed layer reaches its maximum value ( >300 m) in March. Then, in the second phase (April-June), the mixed layer shoals rapidly from April, a large part of the low PV water mass is sheltered from further air-sea interaction by the emerging seasonal ther Based on the in situ XBT and other data sets, analyzing the seasonal cycle of the mixed layer depth (MLD) and using the conservative potential vorticity (PV) as a tool, a clear description of the formation process of the North Pacific Subtropical Mode The forming of NPSTMW reflects well and the ventilation process of the isotherms of the permanent thermocline. The formation process can be divided into the ’ventilation’ phase and the ’formation’. Water (NPSTMW) is presented for explaining the well known ’Stommel Demon’ phase. In the first phase (October-March), with large heat losses at the sea surface from October, the mixed layer deepens and correspondingly, the water mass with low PV emerges and sinks. After continual cooling from October to March, the mixed layer reaching its maximum value (> 300 m) in March. Then, in the second phase (April-June), the mixed layer shoals rapidly from April, a large part of the low PV water mass is sheltered from further air-sea interaction by the emer ging seasonal ther