This research is proposed to find out the relation between the underwater length and the swimming speed of 50 meters backstroke. It used a descriptive research method. The instrument of this research is for measuring the variable which in a form of index text. Video shooting by the camera (DSLR) is done in the needs of underwater length data retrieval. The camera is placed on three different position, such as 5 meters, 10 meters, and 15 meters from the start blocks. The population in this research is 120 swimmers. They are the participants of 50 meters backstroke event in Province Olympic Swimming Championship, which are consists of female and boys age group 1(15-16 years old), age group 2 (13-14 years old), and age group 3 (12-13 years old). Based on the research result, data processing and data analytics, the conclusion of this research is: There is a firm relation between the underwater length and the swimming speed of 50 meters backstroke. By doing the underwater to the maximum, it is not only can reduce the swimming distance of the swimmer, but also can improve the swimmer swimming speed.