The efficacy of radiation therapy can decrease due to the inherent radioresistance of different tumor cells. Gadolinium shows significant potential as a radiosensitivity enhancer due to its high atomic number. In this study, a novel theranostic nanoprobe based on folic acid-conjugated gadolinium-loaded nanodroplets (FA-Gd-NDs) has been introduced for ultrasound imaging (USI)-guided radiation therapy of hepatocellular carcinoma. The ultrasound echogenicity evaluation of NDs, Gd release studies, biocompatibility test of Gd-NDs, colony assay, cellular uptake of NDs via fluorescence microscopy, and flow cytometry analysis were performed on Hepa1-6 cancer and L929 normal cell lines. Our results showed that synthesized NDs significantly enhanced ultrasound signal intensity in PBS solution and agarose gel phantom. MTT and clonogenic assays indicated that Gd-NDs substantially reduced the cell viability and also surviving fraction of Hepa1-6 cancer cells under US and X-ray exposure. Additionally, FA-Gd-NDs exhibited sensitization enhancement factor (SER) of 1.8 after concurrent exposure to US and X-ray. Fluorescence imaging demonstrated more internalization of FA-Gd-NDs into cancer cells in comparison with normal cells. According to flow cytometry results, the Gd-NDs and FA-Gd-NDs uptake by L929 cell line were 20 % and 28 %, respectively, while their uptake by Hepa1-6 cells was 60 % and 94 %, respectively. In conclusion, the synthesized novel theranostic nanoprobe shows great potential for enhancing the efficacy of radiation therapy and enabling ultrasound image-guided radiation therapy of cancers.