Human NAP1L5 is a member of the nucleosome assembly protein (NAP-1) family, until now its function has been completely unknown. In liver cancer, Nap1l5 was suggested as a tumor suppressor gene, and might inhibit cell proliferation. However, it’s reported that many NAP-1 family members promote cell cycle progression. Whether human NAP1L5 promotes or inhibits cell proliferation has been undetermined yet. Here, we found that the proliferation of 293T cells was promoted by Nap1l5 overexpression, and inhibited by knockdown of its expression. Cell cycle analysis showed that compared with control groups, Nap1l5-overexpressed cells had higher percentage of cells in G2-phase, and lower percentage in G1-phase; instead, knockdown of Nap1l5 expression increased the percentage of cells in G1-phase, and decreased the percentage in G2-phase. Our study demonstrated that human NAP1L5 could promote cell proliferation as well as accelerate cell cycle progression, and suggested that the original postulation of Nap1l5 as a tumor suppressor gene was incorrect.