In this study, multifunctional bovine serum albumin (BSA) nanoparticles were fabricated via a green approach with high efficiency. Folic acid (FA) was conjugated to dextran (DEX) through an esterification reaction, and BSA-DEX-FA conjugate was produced by Maillard reaction. Superparamagnetic FeO nanocrystals with a size about 10 nm were loaded into BSA-DEX-FA nanoparticles through a heat treatment which induces BSA gelation. Doxorubicin (DOX) was loaded into FeO/BSA-DEX-FA nanoparticles by a diffusion process. FeO/BSA-DEX-FA and DOX/FeO/BSA-DEX-FA nanoparticles have a size of about 100 nm, good stability, superior transversal R relaxation rate of larger than 360 (mM) s, as well as FA receptor-targeted and magnetically guided functions. FeO/BSA-DEX-FA nanoparticles have excellent biocompatibility. By application of an external magnetic field close to tumor, DOX/FeO/BSA-DEX-FA nanoparticles can effectively enhance the tumor inhibition rate and prolong the life time of H22 tumor-bearing mice, and FeO/BSA-DEX-FA nanoparticles can significantly improve the tumor MRI of KB tumor-bearing mice. This study demonstrates that FeO/BSA-DEX-FA and DOX/FeO/BSA-DEX-FA nanoparticles are suitable systems for tumor diagnosis and therapy.