Assessing the environment inside a tumor would aid the development of an effective therapeutic strategy. Our group has recently developed a pH-activatable nanoparticle containing MR contrast agents, which is called MnCaP micelle. For an in vivo tumor model, a specific and strong enhancement of MR signal in the tumor was obtained after the administration. The enhanced area was agreed with the high lactate region found with 1H-MRS and which corresponds to the hypoxic region inside tumor. We conclude that MnCaP micelle detects hypoxic regions in the tumor clearly and therefore it has potential to provide important information for tumor therapy.