Terahertz frequency comb, which is generated from femtosecond frequency comb, is applied to measure the spatial radiant power density of terahertz source. Two technologies, electro-optic sampling and photoconductive detection, are applied to generate terahertz frequency comb, and the spatial power density of a 100 GHz source is measured with both of these two techniques. The total radiant power is traceable to a standard terahertz radiometer, and the absolute power density of the terahertz source is obtained. The measurement results with both of the two techniques, electro-optic sampling with 800 nm femtosecond optical pulse in free space and photoconductive antenna detection with 1550 nm femtosecond optical pulse in fiber, are analyzed and compared. Moreover, the spatial power density of the terahertz radiation at different distance away from the terahertz source are measured, and the involvement of the terahertz radiation in free space is experimentally studied.