Processed corn or sorghum grain was sieved to obtain particle size fractions and incubated ruminally, by nylon bag technique, to assess the effect of particle size and processing on dry matter and starch disappearance. Dry rolle corn, sieved to obtain fractions with mean particle sizes of 6,000, 3,000, 1,500, and 750 μm and incubated for 2, 4, 6, or 8 h, showed little difference in dry matter disappearance between 6,000 (5.0%) and 3,000, (4.5%) μm corn (averaged over time), but dry matter disappearance increased as particle size fraction was reduced further to means of 1,500 (9.7%) and 750 (18.4%) μm. Starch disappearance displayed a similar pattern. Low dry matter disappearance reflects short incubation times. In a second experiment, corn, steam flaked and then ground, showed much higher dry matter and starch disappearance within each particle size fraction and time than dry ground corn, with particle size appearing much less important in steam flaked corn. In a comparison of ground, high moisture harvested, ensiled corn and dry ground corn, dry matter and starch disappearance were greater in fractions with smaller particles. Moreover, dry matter and starch disappearance were much lower within each particle size in dry ground than high moisture corn. When dry rolled corn was incubated longer (12 and 24 h), average dry matter disappearance increased in fractions with smaller particles as during shorter periods. Particle size had similar effects in sorghum grain. These studies demonstrate processing by steam flaking or high moisture harvesting produces additive effects beyond those of particle size alone. Furthermore, particle size appears to have more influence on dry matter and starch disappearance of unprocessed than processed corn, especially steam flaked corn. Interactions of processing method were with both particle size and incubation time. Processing and particle size effects appear to be important during shorter (2 to 8 h) as well as longer incubation (12 to 24 h).