Abstract Objective No prior studies have used a single sample of bereaved families of cancer patients to compare multiple scales for assessing Complicated Grief. Here, we compare the two measures. Methods We sent a questionnaire to the bereaved families of cancer patients who had died at 71 palliative care units nationwide. Results The analysis included 3173 returned questionnaires. Prevalence of Complicated Grief was 7.8% by Brief Grief Questionnaire (with a cutoff score of 8) and 15.5% for Inventory of Complicated Grief (with a cutoff score of 26). The Spearman’s correlation coefficient between the Brief Grief Questionnaire and the Inventory of Complicated Grief was 0.79, and a ceiling effect was seen for the distribution of the Brief Grief Questionnaire scores. Although 6.4% of respondents scored both 8 or higher on the Brief Grief Questionnaire and 26 or higher on the Inventory of Complicated Grief, only 1.4% scored both 8 or higher on the Brief Grief Questionnaire and <26 on the Inventory of Complicated Grief. In contrast, 9.1% scored <8 on the Brief Grief Questionnaire but 26 or higher on the Inventory of Complicated Grief. Conclusion The prevalence of Complicated Grief was estimated to be higher by the Inventory of Complicated Grief than by the Brief Grief Questionnaire in this sample. Patients with severe Complicated Grief might be difficult to discriminate their intensity of grief by the Brief Grief Questionnaire. Once the diagnostic criteria of Complicated Grief are established, further research, such as optimization of cutoff points and calculations of sensitivity and specificity, will be necessary.