The transmission fluctuations can be used for measurements on particle size and particle concentration in multiphase flows. In Gregory's method, the beam diameter is required to be much larger than the particle diameter, which limits the application seriously. In this work, we introduce a new method of the transmission fluctuation measurement, in which the beam diameter can be less, equal to and larger than the particle diameter. The theoretical analysis proves that, when the beam-to-particle diameter ratio is within the range of 0.05 and 10, the new method is able to achieve satisfying measurement results. However, Gregory’s method is only suitable in the range of 5 and 10. Therefore, this new method nhances the transmission fluctuation measurements greatly.