This invention relates to a method for determining the average surface radius ro and/or the density r of the particles of a nanopowder in a sample levitating in an electropositive plasma with a volume V formed between a first electrode powered by a radiofrequency voltage and a second electrode with a fixed voltage, in particular a ground one, characterized in that: V is the amplitude at the measuring time t of the radiofrequency voltage RF powering the first electrode when the powder is present in the plasma, V is the amplitude of said radiofrequency voltage powering the first electrode in the absence of powder in the plasma, V is the self-polarization voltage of the first electrode at the time t when the powder is present in the plasma, V is the self-polarization voltage of the first electrode in the absence of powder in the plasma, ΔAB is the surface variation of the second electrode in the presence of powder at the time t relative to a situation in which powder is absent, and the values of n and K are determined by means of calibrating by tracing a curve in which ro is a function of ΔAB and of VRF from experimental data obtained from powders having a known particle diameter by measuring VRF, VRF(O), VDC, and VDC(0) and by applying a regression to said curve. n D = 1 4 ⁢ π ⁢ ⁢ K 2 ⁢ V ⁢ ⁢ Δ ⁢ ⁢ A B × ( 1 V RF - 1 V RF ⁡ ( 0 ) ) 2 ⁢ ⁢ n r D = K ⁢ Δ ⁢ ⁢ A B ( 1 V RF - 1 V RF ⁡ ( 0 ) ) n Avec AA : K = ɛ 0 ⁢ C q ⁢ ⁢ V ⁢ ⁢ D n , Δ ⁢ ⁢ A B = δ ⁡ ( t ) δ ⁡ ( 0 ) - 1 1 A M ⁢ ⁢ 0 + 1 A B ⁢ ⁢ 0 , ⁢ δ ⁡ ( t ) = π arccos ⁡ ( V DC V RF ) - 1 et BB ⁢ ⁢ δ ⁡ ( 0 ) = π arccos ⁡ ( V DC ⁡ ( 0 ) V RF ⁡ ( 0 ) ) - 1 AA ⁢ ⁢ with BB ⁢ ⁢ and