The formula of pressure drop coefficient in a radial cyclone dedusting zone is presented. In this formula a real gas friction surface is taken into account. An assumption has been made that the gas velocity distribution coefficient along the radius up to the pattern c(theta)r(n) = const. On the basis of a differential equation, following from the Euler and Bernoulli equations, as well as on the basis of the Barth formula, the equations which describe the velocity field as a function of the distance from the axis of a cyclone and the fortex exponent n were derived. The formula of the pressure drop coefficient in a cyclone dedusting zone was derived on the basis of the rule of conservation of angular momentum. Taking advantage of the literature data on the investigation of twelve cyclones, the values of the friction factors A, the fortex exponents n and of the correction coefficients alpha* were calculated. Dependence of alpha* on F-o/F-c is also presented.