In general, cancer pain is neither fully recognized nor adequately treated. The inadequate pain control is due to failure of accurate assessment of cancer pain. This study was aimed to understand various characters of cancer pain and investigate available assessment scales which have been designed for, or frequently used with people with cancer pain. A total of 23 articles were selected from two different databases. The selected articles were analyzed according to three aspects of initial assessment, patient self-report and assessment of the outcomes of pain management. The characters of cancer pain is complex and includes physical, psychosocial, and spiritual dimension. Also, subjective pain can be classified into at least four specific factors, such as pain intensity, pain affect, pain relief, and pain quality. Based on various classification, the pain assessment scales can be divided into unidimensional or multidimensional. Among the more commonly used clinical tools are numeric rating scales, verbal rating scales, visual analog scales, and picture scales. Above all, in order to assess cancer pain objectively, the clinician must select appropriate assessment instruments which reflect pain definition and clinical purpose.