A simple, new method of measuring velocity and temperature simultaneously without imparting any disturbances to the fluid is developed. The key idea is to use a certain liquid that undergoes thermochromism as a test fluid, the temperature of which can be observed directly by its color change or measured quantitatively by a suitable optical method. If a laser Doppler velocimeter is used, the temperature at the measuring point, in addition to the velocity, can be determined simultaneously and instantaneously by detecting the otherwise discarded intensity of the transmitted light of the laser beams. This method was applied to measurements of distributions of velocity and temperature of a plume in a Hele–Shaw cell. Results were in fairly good agreement with similarity laws derived theoretically in this paper.