In recent decades, extensive attention has been paid to developing anatomic and functional imaging contrast agents that could provide a wealth of complementary bioimaging information. Among them, dual -mode nanoprobes that combine anatomic magnetic resonance imaging (MRI) with functional fluorescent imaging have been mostly used for separated imaging. However, the lack of a machine for simultaneous dual -mode imaging greatly limits further clinical application. One effective strategy is to rationally design MRI contrast agents that own both anatomic and functional MR imaging capability on a single MRI machine, which is highly attractive but remains a great challenge. Herein, ultrasmall NaGdF4@PLL nanodots (NDs) were developed as a novel class of MR contrast agent, which offers a high longitude relaxivity (6.42 mM(-1) s(-1)) for 1'1-weighted MRI and an excellent sensitive chemical exchange saturation transfer (CEST) effect for pH mapping (at +3.7 ppm). Further in vivo animal experiments show the feasibility of NaGdF4@PLL NDs as contrast agents for efficient kidney and brain tumor diagnosis and pH mapping, which will undoubtedly enhance the diagnosis accuracy and is beneficial for disease precaution and prognosis. Different from other complex dual -mode nanoprobes, the as -constructed NaGdF4@PLL NDs enable both anatomic and functional imaging on a single MR machine, which is a simple and cost-effective new approach to realize dual -mode MR imaging and holds great potential for future clinical application.