53BP1 protein is critical for repair of DNA double-strand breaks (DSBs) by non-homologous end-joining repair pathway. 53BP1 binds to dimethylated histones, however, its recruitment to sites of DNA DSBs in addition requires E3 ubiquitin ligases RNF8 and RNF168. The upstream events involve H2AX phosphorylation, RNF8 and RNF168 recruitment, ubiquitination of chromatin proteins, notably histones H2A and H2AX, and, subsequently, recruitment of 53BP1. How protein ubiquitination stimulates 53BP1 recruitment is unclear. Here, we compared the roles of RNF8 and RNF168 and explored the role of H2A/H2AX ubiquitination on 53BP1 recruitment to DNA DSBs. Mild ectopic expression of RNF168 in RNF8-deficient cells rescued 53BP1 recruitment 53BP1 to DNA DSBs. In addition, we show that RNF8 with D443R substitution in RING-domain that mimics RNF168 activity rescued 53BP1 recruitment to DNA DSBs in RNF168-deficient cells. We found that expressing an ubiquitin-H2AX fusion protein rescued 53BP1 recruitment in RNF8 and RNF168-deficient cells. Similar effects were observed when other bulky moieties, including green fluorescent protein, were fused to H2AX N-terminus. Our results suggest that RNF168-mediated H2A/H2AX ubiquitination facilitates 53BP1 recruitment by opening up chromatin.