During recent years, it has been shown that the thioTEPA molecule can play a very important role as an anticancer drug. The present research studies the structural and spectral properties and reactivity of the thioTEPA antitumor agent in confronting the sulfur-donor biomolecules (cysteine and methionine) and DNA purine bases (adenine and guanine). The study was done based on the quantum-mechanical computations. All studied compounds were optimized by B3LYP/6-31+G(d,p) basis set of theory. The IR computations showed no imaginary frequency for all compounds. So, the accuracy of our computational methods was proved. This study indicates that the adenine base has the best reaction with this antitumor drug among all biomolecules. So, the thioTEPA antitumor agent prefers to react with adenine base.