BACKGROUND: Late diagnosis of lung cancer is one of the leading causes of higher mortality in lung cancer patients worldwide. Significant research attention has focused on the use of magnetic resonance imaging (MRI) based nano contrast agents to efficiently locate cancer tumors for surgical removal or disease diagnostics. Although contrast agents offer significant advantages, further clinical applications require improvements in biocompatibility, biosafety and efficacy. RESULTS: To address these challenges, we fabricated ultra-fine Iron Carbonate Nanoparticles (FeCO NPs) for the first time via modified literature method. Synthesized NPs exhibit ultra-fine size (~ 17 nm), good dispersibility and excellent stability in both aqueous and biological media. We evaluated the MR contrast abilities of FeCO NPs and observed remarkable T2 weighted MRI contrast in a concentration dependent manner, with a transverse relaxivity (r2) value of 730.9 ± 4.8 mM Sat 9.4 T. Moreover, the r2 values of present FeCO NPs are respectively 1.95 and 2.3 times higher than the clinically approved contrast agents Resovist and Friedx at same 9.4 T MR scanner. FeCO NPs demonstrate an enhanced T2 weighted contrast for in vivo lung tumors within 5 h of post intravenous administration with no apparent systemic toxicity or induction of inflammation observed in in vivo mice models. CONCLUSION: The excellent biocompatibility and T2 weighted contrast abilities of FeCO NPs suggest potential for future clinical use in early diagnosis of lung tumors.