Abstract Cassava starch/montmorillonite composite films were prepared by casting. This research was focused on the exploitation of chitosan as a compatibilising agent in order to homogeneously disperse the clay particles in a starch matrix. Mixtures of cassava starch, montmorillonite (MMT), chitosan, glycerol as a plasticizer, and distilled water adjusted to pH 3 by acetic acid addition was well mixed with a homogenizer and gelatinized by heating to temperatures of 70–80 °C. The obtained homogeneous starch solution was cast onto an acrylic mold and allowed to dry in open air. X-ray diffraction of the dried film revealed that after treatment with chitosan the interlayer spacing of montmorillonite slightly increased from 14.78 Å to 15.80 Å, providing information that chitosan was too large to intercalate into clay the gallery in order to produce the nanocomposite. Despite the unachieved state of nanocomposite, the evidence from SEM showed that a finer size of clay particles was obtained in the case of the composite film containing chitosan. The results indicated that chitosan due to its hydrophilicity and capability of attaching to the clay surface played a role in compatabilising between starch matrix and montmorillonite. As a result, the starch/MMT composite film at low MMT content exhibited an improvement in tensile properties due to a reinforcement effect. It was also found that the surface hydrophobicity of the composite film increased with an increase in chitosan content. In association with film hydrophobicity, the water vapor transmission rate and moisture absorption were found to decrease with an increase in chitosan content.