In photodynamic therapy (PDT), a target tissue with pre-administered photosensitizer is exposed to laser light. The photochemical process produces reaction oxygen species (ROS), such as singlet oxygen and superoxide, and leads to ultimate cell death. A direct monitoring of ROS production during PDT, thus, may provide important information in both basic science and clinical practice. A cypridina luciferin analogue (FCLA) is a chemiluminescence (CL) probe that selectively detects singlet oxygen and superoxide. In this study, FCLA was used as an optical reporter of ROS produced by photosensitization reaction of Photofrin in Hanks solution and the CL was measured by a photomultiplier system operated at single photon counting mode. By varying the amount of PDT dosage (photosensitizer dose, light irradiation fluence rate) and the amount of FCLA, the intensity of CL were investigated. The results showed the FCLA concentration affects the ratio of the signal to background CL. The decay time of the photosensitized CL was approximately 10 sec., after the excitation source was turned off. In addition, the intensity of the CL-FCLA increased with increasing concentration of Photofrin and fluence rate. The work supported the potential application of FCLA-chemiluminescence probe as a dosimetric tool for PDT.