We have reported three cases in which switching from hydromorphone to oxycodone provided effective analgesia for neuropathic pain caused by cancer. Case 1 was ovarian cancer, case 2 was lung cancer and case 3 was carcinoma of unknown origin. We diagnosed parts of the pain were neuropathic pain caused by lymph node metastasis or bone metastasis of cancer in flow chart of grading system for neuropathic pain diagnosis chart. There are few reports on the effect of hydromorphone on neuropathic pain, the efficacy of oxycodone has been reported in a comparison of hydromorphone-induced morphine and oxycodone. It has been suggested that different effects for neuropathic pain between hydromorphone and oxycodone are due to the activation of GIRK1/ KIR3 channels in the ventrolateral periaqueductal gray matter region. The activation mechanism induces antinociceptive effects by suppressing gammaaminobutyric acid release. In our experience, oxycodone seemed to have a better effect on neuropathic pain than hydromorphone. Therefore, we believe that early use of oxycodone for cancer pain, in which activation-mediated neuropathic pain is considered to predominate in clinical practice, is likely to provide effective pain relief and improve patients' QOL.