OBJECTIVE： To provide reference for the individualized treatment of opioid drugs by investigating the effects of OPRM1 A118G gene polymorphism on opioids dosage in Chinese Han Population patients with cancer pain. METHODS： 66 Chinese Han Population patients with medium and severe cancer pain were selected from a third grade class A hospital. Using fluorescence in-situ hybridization， OPRM1 A118G genotypes were detected to compare the opioids dosage of different genotype in patients with cancer pain. RESULTS： Among 66 patients， distribution frequency of AA， AG and GG genotype were 36.36％， 53.03％ and 10.61％， respectively； those of A and G allele were 62.88％ and 37.12％； the daily dosage of opioids in AA， AG， GG genotype patients respectively were （21.67±13.41）， （42.00±32.18） and （87.14±73.65）mg， with statistical significance （P＜0.001）. Among 39 patients receiving opioids for the first time， the dosage of 6 AA genotype patients and 15 AG genotype patients had been adjusted， with statistical significance （P＝0.026）. After adjusting dosage， daily dosage of AA genotype and AG genotype patients were （16.11±9.16） and（28.57±18.52）mg， with statistical significance （P＝0.011）. CONCLUSIONS： OPRM1 A118G genotype can influence the dosage of opioid in Chinese Han Population patients with cancer pain， and gene polymorphism can be used as evidence for dosage guide of opioids.