With a purpose of improving mainly the water resistivity and related physical and mechanical property of starch biopolymer, citric acid was used to modify the chemical structure of starch by crosslinking the polymer chains. Corn starch films were produced first dispersing 4% (w/w) starch in water, adding glycerol (36% of the weight of starch) and citric acid (20% of the weight starch) successively allowing them to react at different pH. Water absorption, FTIR, DMA, DSC and SEM tests were done to characterize the thin film samples. Results showed that availability of glycerol is a critical factor for the crosslinking and plasticity property of the film. Samples prepared with citric acid crosslinked starch without the inclusion of glycerol were brittle and has easily disintegrated in water as crystals. The crosslinking reaction done under acidic condition was found more effective than the other reactions. Generally, citric acid has effectively crosslinked corn starch polymer molecules reducing the amount of hydroxyl group in their structure. As a result improvement in the water resistant is observed.