Numerical simulations are conducted for the design of a micro thermal mass air flow sensor (MAFS), which consists of a microfabricated heater and thermopiles on the silicon-nitride ($Si_3N_4$) thin membrane structure. It is important to find the proper locations of these thermal elements in the design of MAFS with improved sensitivity. Three heating modes of the micro-heater are considered: constant temperature, constant power and heating pulses. The analyses are focused on the membrane temperature profile near the sensing section. Considered are the practical flow velocities, ranging from 3 m/s to 35 m/s, and the corresponding Reynolds numbers from 1000 to 10000. The results show that one of optimum sensing locations is about $100{mu}m$ away from the microheater. It is concluded that the heating mode and configurations of thermal elements are the main factors for the MAFS with higher sensitivity.