Yang’s (1996) sediment transport theory based on unit stream power is one of the most accurate theories, but in his equations the use of product of slope and critical velocity instead for critical unit stream power is not suitable. Dimensionless critical unit stream power required at incipient motion can be derived from the principle of conservation of power as a function of dimensionless particle diameter and relative roughness. Based on a lot of data sets, this new criterion was developed. By use of this new criteria, Yang’s (1973) sand transport formula and his 1984 gravel transport formula could be improved when sediment concentration is less than about 100 ppm by weight.