We developed a 1H NMR spectroscopic method to quantify CDAs by employing IS and a pseudo-standard material. Owing to the difficulty of obtaining the CDA in pure form and the resemblance of chemical structure and chemical shift with CDA, mesitylene was adopted as a pseudo-standard material. The intensity area ratio of sample to IS was constructed based on NMR spectrums. The relationship between the sample concentration and the peak intensity of 1H NMR showed a high linearity. With this linearity and the standard curve of mesitylene, the cyclic dimer fatty acid was quantified in a mixture containing side products such as acyclic, bicyclic dimer acid, unreacted monomer, overreacted trimer, and oligomers. For the verification of the proposed technique, a known amount of mesitylene was quantified by the curve and compared with the value from the reported equation.