A magnetic nanocomposite consisting of nanoparticlesepolybutylene terephthalate (MNPs-PBT) was electrospun and used as an extracting medium for an on-line mu-solid phase extraction (mu-SPE) ehigh performance liquid chromatography (HPLC) set-up with an ultraviolet (UV) detection system. Due to the magnetic property of the prepared nanofibers, the whole extraction procedure was implemented under an external magnetic field to enhance the extraction efficiencies. The developed method along with the synthesized nanocomposite were found to be appropriate for the determination of trace levels of selected drugs including furosemide, naproxen, diclofenac and clobetasol propionate in the urine sample. The prepared MNPs-PBT electrospun nanocomposite was characterized using the scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and Fourier transform infrared (FT-IR) spectroscopy. The prepared magnetic fibers showed high porosity, which was another driving force for the extraction efficiency enhancement. Major parameters affecting the extraction efficiency of the selected drugs were optimized. The limits of detections (LOD) of the studied drugs were in the range of 0.4-1.6 mu g L(-)1 and the limits of quantification (LOQ) were 1-4 mg L-1 under the optimized conditions. Relative standard deviation (RSD%) for three replicates at three concentration levels of 6, 100 and 400 mu g L-1 were 5.9-8.0% while acceptable linear range with two orders of magnitude was obtained (R-2 = 0.99). The method was validated by the determination of the selected drugs in urine samples and the results indicated that this method has sufficient potential for enrichment and determination of the desired drugs in the urine sample. The relative recovery values were found to be in the range of 78-91%. Implementing the developed oneline mu-SPE method under the external magnetic field induction, led to higher extraction efficiencies for the selected drugs with various diamagnetic properties. (C) 2016 Elsevier B. V. All rights reserved.