Biosafety is a critical issue for the successful translocation of nanomaterial-based therapeutic/diagnostic agents from bench to bedside. For instance, after the withdrawal of clinically approved magnetic resonance (MR) imaging contrast agents (CAs) due to their biosafety issues, there is a massive demand for alternative, efficient, and biocompatible MR contrast agents for future MRI clinical applications. To this end, here we successfully demonstrate the in vivo MR contrast abilities and biocompatibilities of ligand-free FeSn2 alloy NPs for tracking in vivo lung tumors. In vitro and in vivo results reveal the FeSn2 alloy NPs acting as appreciable T2 weighted MR contrast agents to locate tumors. The construction of iron (Fe) on biocompatible tin (Sn) greatly facilitates the reduction of the intrinsic toxicities of Fe in vivo resulting in no significant abnormalities in liver and kidney functions. Therefore, we envision that constructing ligand-free alloy NPs will be a promising candidate for tracking in vivo tumors in future clinical applications.