A novel design and assembly technology was developed for a three-dimensional (3-D) thermal flow sensor based on convective heat transfer to reduce detection error caused by position variation of a sensor inside the flow of narrow and curved geometries, such as coronary artery. The 3-D sensor has three independent sensing elements equally distributed around the catheter tube. This arrangement introduces three independent information channels, and cross-comparisons were utilized to provide accurate flow measurement. The resistance of the sensing elements was measured at approximately 1-1.2 KΩ with the temperature coefficient of resistance (TCR) at 0.086 %/°C. Using a constant-current circuit, the three sensing elements were heated to about 10°C above ambient temperature. Flow testing was implemented in a pipe channel at two positions: on the wall or along the center line. When the sensor was in contact with the pipe, output voltage of the sensing element facing the pipe wall changed less than 0.6 mV. Meanwhile, other two sensing elements which were facing the flow field had a maximum output change of 3.3-3.5 mV. Placing the sensors at the center line of the pipe resulted in an almost equal output voltage change for all sensing elements. Therefore, we demonstrated the capability of 3-D thermal flow sensor for detecting the position of the catheter in the flow channel, thereby providing an accurate flow measurement.