Enteric-coated guar gum microspheres of ornidazole aimed for colon drug delivery have been developed. The influence of core to coat ratio was investigated upon the encapsulation efficiency, swelling, weight loss and drug release behavior. The SEM was used to characterize the surface of these microspheres. Drug–polymer interactions were studied by differential scanning calorimetry and X-ray diffractometry. Enteric coating with Eudragit® S100 enabled maintenance of microspheres integrity until its expected arrival to colon. The in vitro drug release was investigated using USP dissolution rate test paddle type apparatus in different simulated mediums. Only 10 % of drug was release after 5 h while after 8 h (in PBS pH 7.5) a significant increase in percent cumulative drug release (85 %) was observed. In medium containing rat cecal content (pH 7.0) i.e. the amount of the drug released from the formulation was found to be 68 % and ~ 78.3 % with 2 % and 4% w/v cecal matter. Drug release in presence of culture of Bacteriodes ovatus was found to be 77 %. Ornidazole release kinetics corresponds best to zero order model and drug release mechanism was diffusion and swelling controlled. The significance of differences was evaluated by analysis of variance (ANOVA). Differences were considered statistically significant at P < 0.05.