Opioid rotation has been proposed for management of cancer pain. No studies directly investigating dose equivalence between morphine injection (continuous IV administration) and the transdermal fentanyl patch have been reported. Therefore, we examined dose conversion ratios in patients undergoing opioid rotation from morphine injection to fentanyl patches. The subjects consisted of 45 patients admitted to Kitasato University East Hospital. Medical records were consulted to determine the “basic dose of morphine injection immediately prior to rotation” and the “basic dose of fentanyl patch after rotation”. Equivalent doses and conversion ratios obtained with the expression of (daily dose of morphine injection (mg)/daily delivered dose of fentanyl patch (mg)) were determined from the relationship between the data by regression analysis. The regression equation obtained was Y=50.882X-13.96, r²=0.8922, where X and Y are daily doses of morphine injection and fentanyl patch, respectively. Equivalent doses and conversion ratios for daily dose of morphine injection (mg): daily delivered dose of fentanyl patch (mg) (patch dose mg/3 days) were 16.6 mg: 0.6 mg (2.5 mg)=28:1, 47.1 mg: 1.2 mg (5 mg) = 39:1 and 169.2 mg: 3.6 mg (15 mg)=47:1. In other reports, the ratio of morphine vs. fentanyl at 50:1 had no relation to the dose. While the present study suggested that in opioid rotation from low dose, 50:1 is not enough for the fentanyl patch. The dose conversion ratio of morphine injection to fentanyl patch was different at the low doses and high doses of morphine.