The elucidation of protein structures by X-ray crystallography remains the most effectual method to provide accurate structural details at atomic resolution for rational drug design and other biotechnological research studies. Also, emerging applications of protein crystals as ordered nanostructure scaffolds for catalysis, imaging, and drug delivery are attracting much attention. However, the first step of these applications is obtaining high-quality crystals, which is still an obstacle. Successful crystallization requires two steps: nucleation and crystal growth, while the nucleation is a precondition for harvesting the crystal of interest. So controlling protein nucleation may be an alternative breakthrough for this bottleneck. It is well known that nucleants can induce protein crystallization and improve crystal quality, so investigation on the nucleants that can be universally used for any protein crystallization is ongoing. This manuscript reviews the advances that have been achieved using nucleants in protein crystallization and it is a suitable reference for practical crystallization.