A staining technique was developed for evaluating the pore structure of microcrystalline cellulose powders. This technique used two direct dyes as molecular probes and was based on the differences between the two dyes' molecular size and affinity for microcrystalline cellulose. Direct Blue 1 has a smaller molecular size and weaker affinity for microcrystalline cellulose compared to Direct Orange 15, which has a larger molecular size and stronger affinity. We used the ratio of adsorbed Direct Orange 15 to adsorbed Direct Blue 1 after 48 h staining [designated as (O/B)(48)] as the parameter for evaluating the pore structure variations between different microcrystalline cellulose powders. Seven microcrystalline cellulose samples of three different grades-Avicel PH-101, Avicel PH-102 and Avicel PH-103-were analyzed for their (O/B)(48) ratios, and significant differences were found. These differences are present primarily among different grades and secondly among different batches of the same grade. Among the three grades tested, it was found the their (O/B)(48) ratios have the following sequence : Avicel PH-103 > Avicel PH-101 > Avicel PH-102. This suggested that pore structure variations are significant among different microcrystalline celluloses of both different grades and batches, but the degree of variation between different batches is less than that between different grades of microcrystalline cellulose.