Since its introduction over 40 years ago, 5-fluorouracil (5-FU) has remained the only effective chemotherapy option available for the treatment of colorectal cancer (CRC). However, this cytotoxic anticancer drug often causes severe side effects because it does not act selectively on the tumor. It has been reported that the 5-FU showed considerable toxicity when administered by intravenous injections or via alimentary tract. Although, many materials have been developed for carrying 5-FU, there has been no clinically acceptable carrier for 5-FU till now. Montmorillonite, one of the clay minerals, consists of hydrated aluminum silicates with fine grains and large spaces between the layers. Isomorphous substitution of cations is common. In the study, we attempt to intercalate 5-FU into interlayers of montmorillonite through ion exchange. Montmorillonite was purified from crude clays of bentonite in Tai-dong, Taiwan by filtration and sedimentation. Solutions of 5-FU with different concentrations were prepared by dissolving various amounts of 5-FU into 10 ml NaOH solution. Purified montmorillonite powder was soaked in 5-FU solution for a period of time with different pH values and temperatures. In this study, we try to intercalate 5-FU into interlayers of montmorillonite to find out optimum conditions, such as soaking time, temperature, pH value, initial 5-FU concentration, etc., to prepare composites of 5-FU and montmorillonite (5-FU/mont). UV, SDT, FTIR, XRD are used to characterize the 5-FU/mont composite. From the results. 5-FU was successfully intercalated into the interlayer of montmorillonite both by free surface absorption and OH replacement. The optimum condition for 5-FU/mont preparations is 1.185 wt% of 5-FU as initial concentration under a pH value of 11.6 at a temperature of 80 degrees C and a soaking time of 2 h. The total amount of 5-FU in montmorillonite is about 87.5 mg for each gram of montmorillonite, which can be proved by thermal gravimetric analysis. The composite of 5-FU/mont is expected to achieve in situ release for colorectal cancer therapy in future applications.