A magnetic carboxyl-modified nanodiamond (MCND) was used for the magnetic solid phase extraction of Pb(II), Cd(II), and Co(II) prior to their quantification by flame atomic absorption spectrometry. The external magnetic field was used to separate the magnetic solid phase from the analyte solution in order to avoid sample loss during filtration and centrifugation. Various factors affecting the extraction efficiency of the adsorbent such as solution pH, adsorbent dose, sample voltume, temperature, matrix effect, and desorption condition were optimized. The analytical parameters of the limit of detection (Pb: 0.99 mu g L-1, Co: 1.73 mu g L-1, Cd: 0.15 mu g L-1), preconcentration factor of 20 for the analytes, and percent relative standard deviation (Pb: 0.5%, Cd: 0.6%, Co: 0.6%) were calculated under optimized experimental condition. The kinetic data show that this adsorption process follows pseudo-second order kinetics. The thermodynamic data show that this adsorption process was feasible and exothermic in nature. The method was validated by applying the method to certified reference materials TMDA 64.2 and SPS-WW2 and natural water samples with satisfactory recovery results.