Excited-state intramolecular proton transfer (ESIPT) reactions of three ortho-hydroxylated oxazolines, 2-(4,4-dimethyl-4,5-dihydro-oxazol-2-yl)-phenol (DDOP), 4-(4,4-dimethyl-4,5-dihydro-oxazol- 2-yl)-[1,1΄-biphenyl]-3-ol (DDOP-C₆H₅) and 4-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxy-benzonitrile (DDOP-CN), have been systematically explored by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Two stable configurations (enol and keto forms) are found in the ground states (S₀) for all the compounds while the enol form only exists in the first excited states (S₁) for the compound modified with electron donating group (-C₆H₅). In addition, the calculated absorption and emission spectra of the compounds are in good agreements with the experiments. Infrared vibrational spectra at the hydrogen bond groups demonstrate that the intramolecular hydrogen bond O(1)−H(2)···N(3) in DDOP-C₆H₅ is strengthened in the S₁ states, while the frontier molecular orbitals further reveal that the ESIPT reactions are more likely to occur in the S₁ states for all the compounds. Besides, the proton transfer potential energy curves show that the enol forms can barely convert into keto forms in the S₀ states because of the high energy barriers. Meanwhile, intramolecular proton transfer of all the compounds could occur in S₁ states. The ESIPT reactions of the ortho-hydroxylated oxazolines are barrierless processes for unsubstituted DDOP and electron withdrawing substituted DDOP-CN, while the electron donating substituted DDOP-C₆H₅ has a small barrier, so the electron donating is unfavorable to the ESIPT reactions of ortho-hydroxylated oxazolines.