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Vitamin C (ascorbic acid) is one of the water-soluble vitamins. It can be found widely in raw fruits, vegetables and other food products, such as juices. There is an increasing in the awareness of the health benefits of vitamin C as an essential nutrient and antioxidant. Therefore, there is a need to quantify vitamin C present in foods and pharmaceutical products for quality control and security purposes. In this study, the feasibility of using ATR-THz spectroscopy for quantitative study of vitamin C solution will be investigated. A number of 33 samples of vitamin C solution were prepared by dissolving vitamin C in distilled water in the concentration range 3-21%. The spectra of vitamin C solution were acquired using a terahertz-based Fourier Transform spectrometer (FARIS-1S, JASCO Co., Japan). This spectrometer was equipped with a high pressure of mercury lamp as light source and a pyroelectric sensor made from deuterated L-alanine triglycine sulfate (DLATGS) as detector. The spectral absorbance information from 450 cm-1 to 20 cm-1 was used for further analysis. In quantitative study, partial least squares regression (PLSR) was used to develop calibration model for determination of vitamin C concentration. Full cross-validation was used to validate the model. The result showed that using raw spectra, validation resulted in acceptable RPD value (RPD = 3.53). This study showed that ATR-THz spectroscopy method provides useful technique to quantify vitamin C concentration especially in monocomponent solution.