Background: Duloxetine, which is used to treat major depressive disorder and diabetic peripheral neuropathic pain, can be analyzed with different analytical methods, including high performance liquid chromatography-tandem mass spectrometry, electrophoresis with fluorescence detection, HPLC with fluorescence detection and so on. However, these methods need time consuming sample preparations or special and expensive instruments. It is nessecary to develop a simple, rapid, sensitive and cost effective method for the determination of duloxetine in biological fluids.Methods: A combination of magnetic dispersive solid-phase extraction procedure with high performance liquid chromatographic-ultraviolet detection (HPLC-UV) was developed for the determination of duloxetine in human plasma and urine samples. Magnetic Fe3O4 nanoparticles grafted oxidized-multiwalled carbon nanotube (MNPs-MWCNTs) was used as an efficient adsorbent during the preconcentration step. Results: Under the optimized conditions, the calibration curve was linear over the concentration range of 10-2500 ng mL-1 (R2 = 0.9991). Validation experiments revealed that the optimized method has high percent recovery (96%), high enrichment factor (120), good precision as RSD% (intra-day: 3.28, 3.44; inter-day: 4.29, 4.51 for urine and plasma, respectively, C=1000 ng mL-1, n=6) and low detection limit (2.1 ng mL-1). Conclusion: Obtained results indicated that the proposed method has advantages of convenience, good sensitivity and high efficiency, and it was feasible for the determination of duloxetine in biological fluids.