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Hydrophobic carboxymethyl starch (HCMS) was successfully synthesized, and major factors affecting esterification were systematically investigated. The physicochemical properties of HCMS were determined through FTIR spectroscopy, scanning electron microscopy (SEM), and viscosity analysis. The results indicated that the suitable parameters for the preparation of HCMS from carboxymethyl corn starch (CMS) were as follows: temperature, 35°C; reaction time, 60 min; benzoyl chloride (BC), 2.5 (in mole proportion to CMS); and sodium hydroxide (NaOH), 1.0 (in mole proportion to BC). Under these conditions, the degree of substitution (DSBz) was approximately 0.85 and the reaction efficiency (REBz) was approximately 43%. FTIR analysis revealed that benzoyl groups were grafted onto the CMS chain units. SEM images showed that HCMS granules exhibited slightly rough surfaces and their edges lost some definition; their sides were porous. The HCMS displayed good solubility and hydrophobicity. The selected HCMS (DSBz, 0.31) drilling fluids provided a remarkably effective reservoir protection in the cores, and its permeability recovery value reached 90%.