The present invention relates to an agent for photodynamic therapy (PDT) containing porous silicon (PSi) and a method of quantitative measurement of reactive oxygen species (ROS) produced therefrom. Particularly, it is expected that the present invention will contribute to the development of cancer treatment without side effects by suggesting PSi, a safe and reliable new cancer drug that can destroy cancer cells through generating sufficient heat with little ROS produced if the PSi is used as nanobomb for a new PDT that kills cancer cells with heat or explosion. In addition, it is expected that the present invention will provide a reliable and reproducible method for quantitative measurement of a very small amount of ROS produced therefrom through an XRD analysis.