Iron isotopic compositions of siderite minerals from the carbonate rocks in the Mansa siderite deposit of Mengla County in Yunnan Province were studied for the first time in order to discuss the genesis and ore-forming environment of the deposit. Fe isotopes of siderite from the Mansa deposit show a relatively small negative range: δ⁵⁶Fe values of siderite range from -0.337‰ to -0.201‰ (averagely -0.261‰), whereas δ⁵⁷Fe values from -0.518‰ to -0.289‰ (averagely -0.380‰), implying that the Fe isotopic compositions are rather homogeneous and enriched in light iron isotope. Detailed Fe isotope studies of the Mansa siderite deposit show a close relationship between metallogenesis and hydrothermal process, and hence the deposit was formed in a partial submarine environment with rapid accumulation after the effusion of the ore-bearing hydrothermal solution.