Upconversion nanoparticles with special fluorescence and magnetic properties have been considered an alternative contrast agent for multiple bio-imaging techniques. It is important to understand the effects of the surface properties and dosage of upconversion nanoparticles on both the magnetic resonance (MRI) image and the photoluminescence spectrum. Here, NaGdF4: Er3+, Yb3+ upconversion nanoparticles (UCNPs) modified with amine functional group were produced through a one-pot thermal decomposition. The average length of the cubic UCNPs is estimated at 53 +/- 13 nm. The effect of the dosage of amine modified UCNPs on the MRI image is investigated. The T-1 and T-2 relaxivities of the amine modified UCNPs in agarose gel at 3 T are r(1) = 6.79 +/- 0.14 and r(2) = 17.0 +/- 0.18 (mmol/L)(-1) s(-1), which are comparable to the relaxivities of commercially available MRI contrast agents. In addition, the photoluminescence of the amine modified UCNPs at low concentrations < 150 mu g/mL are further investigated with the excitation wavelength (lambda(ex)) at 980 nm. The internalization of the amine modified UCNPs cultured with human umbilical vascular endothelial cells (HUVEC) is observed by the fluorescence imaging. Meanwhile, T-1-weighted MRI imaging of HUVEC cells treated with amine modified UCNPs at 10 mu g/mL can be obtained. No significant toxic effect on cells is found when the concentration of the amine modified UCNPs is < 300 mu g/mL. This study indicates that a low concentration of amine-modified NaGdF4: Er3+, Yb3+ UCNPs can be used as the contrast agent for both fluorescence imaging and magnetic resonance imaging.