Tannic acid, a hydrolysable tannin, exists commonly in food plants. Tannic acid has already been shown various anticancer mechanisms such as inhibiting the proliferation, inducing a higher apoptotic rate and slowing down the metastasis of different cancers. Moreover, tannic acid was reported to reduce the side effects caused by chemotherapeutics on patients. But whether the tannic acid can improve the treatment of oxaliplatin on colorectal carcinomatosis has yet been studied. In this study, we developed an injectable drug delivery system by physical incorporation of oxaliplatin (OXA) and tannic acid (TA) polymeric nanoparticles (OXA/TA NPs) into a thermo-sensitive hydrogel, OXA/TA NPs-hydrogel (OXA/TA NPs-H). The OXA/TA NPs-H was injected into the peritoneal cavity for the treatment of colorectal peritoneal carcinoma. Firstly, a water-in-oil-in-water double-emulsion (w/o/w) method and solvent-evaporation procedure were used in the preparation of the biodegradable OXA/TA NPs. Then, we prepared the biodegradable thermo-sensitive poly(3-caprolactone) (PCL)-10R5-PCL (PCLR) hydrogel with a low critical solution temperature (LCST) which undergoes gelation process at body temperature. Transmission electron microscopy (TEM) showed the spherical profile of OXA/TA NPs. Fourier-transform infrared (FTIR) spectra demonstrated that OXA and TA were both encapsulated into the OXA/TA NPs. In this study, intraperitoneal application of OXA/TA NPs-H restricted the growth of CT26 peritoneal colon cancer in vivo, improved the quality of life and prolonged the survival time of the model mice. Our study suggested that OXA/TA NPs-H might have potential application in the treatment of colorectal cancer. Copyright © 2018. Published by Elsevier B.V.