A new magnetic core-shell multiwalled carbon nanotube@nanodiamond nanocomposite was synthesized, characterized and used as magnetic solid-phase extraction material. The usability of material was checked on the separation and pre-concentration of quercetin in tea samples by magnetic solid-phase extraction and high-performance liquid chromatography with diode array detector system. There was a linear relationship between chromatographic area and concentration of quercetin in the range of 5.0-600 ng mL(-1). The limit of detection and quantification limits were 1.46 and 4.91 ng mL(-1), respectively. The developed method has been applied to determine quercetin at trace levels in tea samples.