This paper presents an application of Bernoulli's law to check the Betz coefficient (the limit value of converting from wind energy to rotational kinetic energy of wind turbine). The calculation method is based on the principle of conservation of mass, conservation of energy, and the relationship between the pressure and speed of air flow moving through a turbine, which is placed in a flow tube. The results show that the limit value of conversion from wind energy to rotational kinetic energy of turbine obtained by our method is consistent with the Betz value published previously.