Synthesis of star polymers via arms-first using atom transfer radical polymerization (ATRP) and gelation in the star-polymer system were simulated using a Monte Carlo method based on the dynamic lattice liquid model (DLL). The arms-first, one-pot star synthesis consists of growing linear chains first and at specific monomer conversion subsequent addition of the cross-linker, which leads to formation of stars and/or gels. Simulations of polymerizations in which the cross-linker was added at various monomer conversions and various dilutions were carried out. The distribution of the number of arms in all cases was quite broad and the number of stars with a given number of arms N-a decreases with increasing N-a. At low dilution (more concentrated solutions), formation of stars with hundreds of arms and then gelation are observed. The gel point was higher the higher was the monomer conversion at which the crosslinker was added. In more diluted systems, gelation was not observed and only star polymers were obtained. Stars with relatively few arms dominated in such a case. Intrachain cross-linking (cyclization) is an important factor, decreasing the number of effective cross-links. The gel points were much lower and star yield increased, if intrachain cyclization was not allowed. (C) 2013 Elsevier Ltd. All rights reserved.