Seven synthetic dyes (amaranth, ponceau 4R, sudan red 1, tartrazine, sunset yellow FCF, fast green FCF and brilliant blue FCF), commonly used as colorant in foodstuffs and soft drinks, were investigated on monolithic C18 column by high performance liquid chromatography. The compositions of mobile phase containing methanol-water in acetate buffer at different pH were optimized at the maximum detection wavelength of each synthetic dye. Relative standard deviations of retention time, peak area and peak height for seven synthetic dyes were 0.18-0.83, 0.44-3.49 and 0.49-3.56%, respectively. The detection limit (at a signal-to-noise ratio of 3) for amaranth, ponceau 4R, sudan red 1, tartrazine, sunset yellow FCF, fast green FCF and brilliant blue FCF were 3.38, 2.89, 2.62, 1.92, 3.38, 1.23 and 1.56 μg/L, respectively. The optimal methods were applied to identify and determine the synthetic dyes in seven foodstuffs and four soft drinks. The results showed that some samples contained synthetic dye at a higher concentration than the maximum legal limit in foods and soft drinks of Thailand.