Magnetic-activated carbon cloth (MACC) was synthesized, characterized, and used as magnetic adsorbent in magnetic solid-phase extraction of quercetin prior to UV-visible spectrophotometric determination. Different parameters such as pH, amount of MACC, effect of volume, and elution time were optimized for the determination of quercetin using UV-visible spectrophotometer at 370nm. In this procedure, quercetin molecules in aqueous sample phase were adsorbed on the 10mg of the adsorbent at pH 4.0 and desorbed with acidic methanol solution by using vortex mixer. Limit of detection (LOD), limit of quantification (LOQ), and relative standard deviation were found as 1.4ngmL(-1), 4.8ngmL(-1) and lower than 10%, respectively. The applicability of the developed magnetic solid-phase extraction method was proved on onion samples.