Enhancing　the　generation　of　reactive　oxygen　species　(ROS)　is　an　effective　anticancer　strategy.　However,　it　is　a　great　challenge　to　control　the　production　and　to　image　ROS　in　vivo,　both　of　which　are　vital　for　improving　the　efficacy　and　accuracy　of　cancer　therapy.　Herein,　an　activatable　semiconducting　theranostic　nanoparticle　(NP)　platform　is　developed　that　can　simultaneously　enhance　ROS　generation　while　self-monitoring　its　levels　through　ratiometric　photoacoustic　(PA)　imaging.　The　NP　platform　can　further　guide　in　vivo　therapeutic　effect　in　tumors.　The　theranostic　NP　platform　is　composed　of:　(i)　cisplatin　prodrug　and　ferric　ion　catalyst　for　ROS　generation,　a　part　of　combination　cancer　therapy;　and　(ii)　a　ratiometric　PA　imaging　nanoprobe　consisting　of　inert　semiconducting　perylene-diimide　(PDI)　and　ROS　activatable　near-infrared　dye　(IR790s),　used　in　ratiometric　PA　imaging　of　ROS　during　cancer　treatment.　Ratiometric　PA　signals　are　measured　at　two　near-infrared　excitation　wavelengths:　680　and　790　nm　for　PDI　and　IR790s,　respectively.　The　measurements　show　highly　accurate　visualization　of　(OH)-O-center　dot　generation　in　vivo.　This　novel　ROS　responsive　organic　theranostic　NP　allows　not　only　synergistic　cancer　chemotherapy　but　also　real-time　monitoring　of　the　therapeutic　effect　through　ratiometric　PA　imaging.