The coarsening of γ′ precipitate in Fe-Ni-Co based alloys with different contents of P (Mn) was investigated by TEM. For both experimental alloys, the growth kinetics of γ′ particles were in good accordance with the prediction of the Lifshitz-Wagner theory of diffusion-controlled coarsening with the exception of a little larger cut off value in the histograms of the γ′ precipitate. Compared with the usual alloy, the alloy with a lower content of P (Mn) showed that the γ′ particles exhibited a lower growth rate, and the density and size distribution of the precipitate were found to be more homogeneous.