Antibiotic-resistant bacterial cell infection is a serious problem in many countries. Reactive oxygen species (ROS), which is toxic to bacterial cells, is one of reactive species responsible for the photoactivity of semiconductor nanoparticles. In this study, we have prepared highly efficient antibiotical colloidal nanocrystal quantum dots (QDs). The bandgap energy of cadmium-free QDs (indium phosphide quantum dots, InP QDs) was tuned their size. The cadmium-free QDs were treated to bacterial cells (drug-resistant Escherichia coli, MG 1655 strain) and the bacterial viability was analyzed to confirm the QD-induced bacterial cell death. The cadmium-free QDs efficiently attacked and growth-inhibited the bacterial cell, but had less effect on the mammalian cells. The cadmium-free QDs could be used for clinical phototherapy to care the bacterial infections.