Abstract: A Laser Doppler Velocimetry system containing a particle generator, a particle collector and a motorized precision translation stage is built to diagnose the fluid fields of a vertical flow burner. The free jet flame and impinging jet flame are investigated, both for a single nozzle (200W power) and a coaxial dual-nozzle (1200W power). An adaptable signal to noise ratio (SNR) threshold is analyzed and employed for post-processing. The experimental data shows high repeatability and accuracy in multiple measurements. For impinging jet cases, the Reynolds numbers (Re) of low power and high power flame are 1200 and 7200, respectively. The mean velocity vectors and contours are sketched from the measurements at different axial and radial positions, displaying the main characteristics of the impinging jet flame. Meanwhile, a peak of the horizontal velocity occurs roughly at one-nozzle-diameter distance departed from the nozzle axis in the near-wall region. This feature possibly provides an explanation for the mechanism of the secondary peak of the heat transfer captured in previous literatures. For the cases of coaxial jet, a mixing region exists between the outer annular jet and the core jet:the mixing zone is gradually damped with the development of the free jet flame, whereas radially expanding in the impinging flame driven by the high-pressure stagnation region.