Transmission signal of radiation in suspension of particles performed with a high spatial and temporal resolution shows significant fluctuations, which are related to the physical properties of the particles and the process of spatial and temporal averaging. Exploiting this connection, it is possible to calculate the particle size distribution (PSD) and particle concentration. This paper provides an approach of transmission fluctuation spectrometry (TFS) with variable spatial averaging. The transmission fluctuations are expressed in terms of the expectancy of transmission square (ETS) and are obtained as a spectrum, which is a function of the variable beam diameter. The reversal point and the depth of the spectrum contain the information of particle size and particle concentration, respectively. Transmission signal of radiation in suspension of particles performed with a high spatial and temporal resolution shows significant fluctuations, which are related to the physical properties of the particles and the process of spatial and temporal averaging. size distribution (PSD) and particle concentration. This paper provides an approach of transmission fluctuation spectrometry (TFS) with variable spatial averaging. The transmission flows are expressed in terms of the expectancy of transmission square (ETS) and are obtained as a spectrum, which is a function of the variable beam diameter. The reversal point and the depth of the spectrum contain the information of particle size and particle concentration, respectively.