Photodynamic therapy (PDT) utilizes photon energy to activate a pre-administered photosensitizer drug in tissue to achieve a localized tumor control. PDT cell killing mechanism is directly related to the reactive oxygen species (ROS) produced during the photochemical reactions. Conventional PDT dosimetry evaluates distributions of the photosensitizer drug, photon propagation and absorption, and availability of molecular oxygen in the target tissue. Yet, the ultimate bullet for the damaging effect is ROS. An evaluation of ROS production during PDT should provide a more direct marker for PDT. Fluoresceinyl Cypridina Luciferin Analog (FCLA) is a chemiluminescence probe that specifically interacts with ROS (singlet oxygen and/or superoxide). The work is a preliminary investigation of the feasibility using FCLA as a means to evaluate ROS production in PDT.