Background/Aim: Glioblastoma, also known as glioblastoma multiforme (GBM), is the most aggressive type of primary brain tumor and a cornerstone in its treatment is radiotherapy (RT). However, RT for GBM is largely ineffective at clinically safe doses, thus, the study of radiosensitizers is of great significance.Materials and Methods: With accumulating evidence for the anticancer effect of compounds from cranberry, this study was designed to investigate if cranberry extract (CE) sensitizes GBM to RT in the widely used human glioblastoma cell line U87. We utilized clonogenic survival assays, cell proliferation assays, and caspase-3 activity kits. Potential proliferative and apoptotic molecular mechanisms were evaluated by reverse transcription-polymerase chain reaction.Results: We found that CE alone had little effect on the survival of U87 cells. However, RT supplemented by CE significantly inhibited proliferation and promoted apoptosis of U87 cells when compared with RT alone. The proliferation-inhibitory effect of RT/CE might be attributable to the up-regulation of p21, along with the down-regulation of cyclin B and cyclin-dependent kinase 4. This pro-apoptotic effect might additionally be attributable to the down-regulation of surviving.Conclusion: These results warrant further study of the potential radiosensitizing capacity of CE in glioblastoma and other cancer types.