Photodynamic therapy for deep-lying lesions needs an appropriate imaging modality, precise evaluation of tissue oxygen and an effective photosensitizer. Gadolinium based metalloporphyrins Gd(III)-HMME is proposed in this study as a potential multifunctional theranostic agent, as photosensitizer, ratiometric oxygen sensor and MRI contrast agent. The time resolved spectroscopy revealed the luminescence peak of Gd(III)-HMME at 710 and 779 nm with a lifetime of 64 μs in oxygen-free methanol to be phosphorescent. This phosphorescence is strongly dependent on dissolved oxygen concentration. Its intensity in oxygen saturated methanol solution is 21% of that in deoxygenated solution. The singlet oxygen quantum yields ΦΔ of HMME and Gd(III)-HMME in air saturated methanol solution were determined to be 0.79 and 0.40 respectively using comparative spectra method. These phenomena indicate that the oxygen sensibility and production of singlet oxygen of Gd(III)-HMME can fulfill the requirement of PDT treatment.