Corn starch was crosslinked with sodium trimetaphosphate (STMP) in solid state under microwave irradiation to improve the degree of substitution (DS) and decrease the reaction time. The effects of modification factors on the crosslinking of starch were systematically studied. It was found that the microwave irradiation provided a convenient and efficient method to prepare crosslinked starch with sufficient DS. The DS value of crosslinked starches obtained by microwave irradiation was significantly improved and the reaction time was also dramatically decreased to achieve an appropriate DS value by microwave irradiation, when compared with using traditional method. The high microwave power provided faster reaction kinetics to obtain the desired DS value at an identical reaction condition. If the reaction temperature is lower than the decomposition temperature of crosslinked starch, the higher DS value can be achieved at the higher microwave power. It was detected that the DS data obtained by reacting intermittently was higher than those by reacting continuously. It was also noticed that DS data varied drastically with the increasing of the amount of STMP and sodium carbonate. The swelling ratio of crosslinked starch in water was lower than that of native starch because of the non-gelatinization of highly crosslinked starch. SEM analysis indicated that the crosslinking of corn starch by microwave irradiation caused no significant changes in the microstructure of starch granules. Light microscope analysis also showed the uniformity of so-prepared crosslinked corn starch applying microwave irradiation. (c) 2006 Wiley Periodicals, Inc.