The research work presented in this thesis focuses on the synthesis and tailoring lanthanide nanomaterials for nanomedicine, namely bio-imaging and cancer ablation. Owing to the sharp, intense up-conversion emission in wavelength ranging from ultra-violet to near-infrared, and unique magnetic properties, lanthanide doped nanomaterials have attracted strong research interests in nanomedicine. Ytterbium enriched NaYF4：Yb3+, Er3+/NaDyF4: Yb3+ nanocrystals were first prepared as a powerful dual-mode contrast agent in fluorescence imaging and magnetic resonance imaging (MRI) by rational integration of different lanthanide ions within a single nanocrystal. The “poisonous” effect of Dy3+ on up-conversion emitters Er3+ has been successfully circumvented and the transverse relaxivities of the nanocrystals are much higher than other T2 contrast agents, indicating their suitability as contrast agents in up-conversion fluorescence and MRI. NaDyF4:Yb3+/ NaGdF4:Yb3+, Er3+ nanorods (NRs) were then synthesized. The NRs show excellent dark T2 contrast enhancement due to the presence of Dy3+ ions. Interestingly, tunable positive and negative T1 enhancement can be achieved by using different MRI sequences. They also show good up-conversion fluorescence, which indicate their potential as a new generation dual mode “smart” contrast agents in the area of optical/MRI bimodal imaging. Near-UV and blue up-converting NaYF4:Yb3+, Tm3+ NCs have been optimized and further used to excited CdSe QDs in order to achieve tunable up-converting multicolor emission in core-shell NaYF4:Yb3+, Tm3+-CdSe nanocrystals. These nanocrystals are potential candidates as multicolor probes and contrast agents in imaging, fluorescence-based assay, optical barcodes for identifying analytes in multiplexed assays and chemosensing or biosensing. These nanocrystals open up new avenues as multicolor fluorescence in biolabeling in and biodetection applications.