The Loop formation of a polymer chain is a fundamental process for many biological functions and has been studied for several decades. The cellular environment is highly crowded with macromolecules where the volume occupancy can reach up to 30~35%. In this study we analyze the effects of molecular crowding on the loop formation of the chain ends by usingthe Langevin dynamics simulations. We demonstrate, in particular, non-trivial effects of the size of the crowding molecules. Namely, for smaller crowders the increase of viscosity dominates, whereas for bigger crowders the confinement effect prevails. Thus the polymer loop formation can be impeded or facilitated depending on the size of the crowders. We also discuss our results in connection with recent experiments.