Oat kernel size distributions are important to the oat milling industry because size separation of kernels is routine in oat milling. Dehuller rotor speeds are set in order to deliver the optimal mechanical stress to different kernel size streams for dehulling. In this study, size distributions were evaluated by digital image analysis in 10 cultivars grown in eight environments. Observed distributions were compared with quality characteristics and with panicle characteristics and spikelet type frequencies. Size distributions within samples, as evaluated from individual kernel image areas, tended to depart from normal distributions and graphical depictions of data frequently resembled bimodal populations. A statistical test to compare a bimodal distribution with a normal distribution indicated that a bimodal model was more effective at describing the distributions. Panicle analysis indicated that two-kernel spikelets were the most abundant spikelet type found. Because two-kernel spikelets consist of one larger kernel and one smaller kernel, it is likely that the root of the bimodal distribution can be attributed to these spikelets. Although some departures from the mixture of two normal distributions can be attributed to the occurrence of one- and three-kernel spikelets, many of these departures must be attributed to other sources of variation in oat kernel size. Key words: Oats, panicle, kernel size, spikelets