Process for polymerization and, in particular, to a process for start-up of a gas phase fluidized bed polymerization reaction involving one or more monomers using a catalyst in a reactor. The process includes providing a start-up bed of particles in the reactor in the presence of the one or more monomers but in the absence of the catalyst, introducing at T0 catalyst to the reactor to initiate reaction and start-up polymer production at a time Ti, Ti being between 1 and 6 hours after T0. The value of Xi is less than a threshold value, wherein: Xi=Cumulative production in the time period T0 to Ti/(Cumulative catalyst injection the time period T0 to Ti*PiC2=), where PiC2= is the ethylene partial pressure in the reactor at the time Ti, subsequently stopping the injection of catalyst, and taking a corrective action to address the low value of Xi.