Method for determining the density (D) of a multiphase fluid (F) flowing through a pipe (1) and containing a gas (G) which determines a fluctuation of the actual instantaneous density of the fluid (F), said method comprising the following steps: defining a series of consecutive time periods (S0, S) which are shorter than the time length corresponding to the highest frequency-component of the fluctuation of the actual instantaneous density; conveying through a section (2) of the pipe (1) a photon beam (3) suited to undergo an attenuation while crossing the fluid (F); counting the actual number (A) of photons of the beam (3) which cross the pipe (1) during a first time period (S) of the series; calculating a corrected count (C) as the weighted average value of the actual count (A) and of the corrected count (C0) calculated for the preceding time period (S0); converting the value of the corrected count (C) into a corresponding value of the density (D) of the multiphase fluid (F) for the first time period (S) through a predefined function.