The accuracy of mass air flow rate measurement by thermal mass air flow sensors (MAFS) is critical to computer-controlled engine operations. However, accurate measurements are usually limited by the principle and the size of the thermal anemometer. Detailed flow field information at the inlet plane of a MAFS is needed to improve MAFS design and operation.In the present investigation, point velocity measurements (mean and fluctuating components) along the inlet plane of a MAFS are conducted by using a laser Doppler anemometer system. The effects of the size of the thermal elements on the accuracy of the MAFS mass air flow measurements are explored. The effects of a dust-aged filter and a partially blocked filter on the flow structure are also examined. It is found that the velocity distributions are complicated at the inlet plane and are strongly influenced by the geometry and by the filter blockage effects. The experimental results are useful to mass air flow sensor design and operation, and to the verification of the accuracy of numerical calculations.