A micromachined thermal mass flow sensor comprises a high mechanical strength polyimide film as a supporting layer of suspending membrane. The polyimide film provides superior thermal insulating properties to reduce the power consumption of device. Due to the tendency of humidity absorption, the polyimide suspending membrane is double side passivated on both top and bottom surfaces to sustain its long term stability from rush and humid working environment. A thin layer of silicon dioxide deposited by plasma enhanced chemical vapor deposition is overlaid between the silicon nitride and polyimide film to enhance the adhesion property of passivation layers to polyimide surface. With such embodiments, a sturdy and robust micromachined thermal mass flow sensor with high measurement accuracy could be formed.