The concept of using a micro-reactor tool for studying single particle polymerization is demonstrated in the present study. The micro-reactor is equipped with a video microscope for capturing the growth of polyethylene-particles during polymerization. The gas-phase polymerizations are done in conditions very close to industrial set points. As catalyst a standard 4th generation Ziegler–Natta system has been used. From the obtained particle growth curves the rate of polymerization is calculated and related with SEM pictures to allow better understanding of the gas-phase polymerization of ethylene. Different Al/Ti ratios and reaction temperatures are examined and explain the behavior of growing polyethylene-particles in a satisfying way.