A new method to evaluate the critical cooling rate, R c of Fe-based metallic glass alloy was proposed and discussed. [(Fe 0.5Co 0.5) 0.75B 0.2Si 0.05] 96Nb 4 alloy particles were prepared with narrow size distribution and high sphericity by Pulsated Orifice Ejection Method in Ar, He and 50%Ar+50%He mixed atmosphere, respectively. Phase transition of a particle from amorphous to amorphous-crystalline and fully crystalline occurred with the increase of particle diameter. R c of the formation of fully amorphous phase was estimated to be in the range of 700-1100 K/s, lower than that measured by time-temperature transformation diagram of bulk metallic alloy. No change of R c occurred in Ar, He or 50%Ar+50%He mixed atmosphere, which proved it an effective method to evaluate the critical cooling rate of Fe-based metallic glass alloy.