A separation-preconcentration method is presented for the solid phase extraction (SPE) of trace thorium(IV) as its arsenazo III complex in environmental samples on multi-walled carbon nanotubes (MWCNTs) by UV-Vis spectrophotometry. The influence of pH of the sample solutions, amount of arsenazo III, eluent type and volume, and sample volume were studied. The effects of the matrix ions were also examined. The detection limits (LOD) and quantification limits (LOQ) of 0.035 and 0.12 mu g/L, respectively, were obtained. An enrichment factor of 40 was achieved for Th(IV) and the relative standard deviation was < 5 %. The presented method was applied for the determination of thorium in an ore sample with satisfactory results.