Kinesins are a large gene family involved in many basic processes of plant development. However, the number of functionally identified kinesins in rice is very limited. Here, we report a functional characterization of Brittle Culm12, a gene encoding a Kinesin-4 protein. bc12 mutants display a dwarfism phenotype that results from a significant reduction in cell number and a brittleness that is due to alteration of cellulose microfibril orientation and wall composition. BC12 is expressed mainly in tissues undergoing cell division and secondary wall thickening. In vitro biochemical analyses verified BC12 as an authentic motor protein. This protein was found to be present in both nucleus and cytoplasm and to associate with some microtubule arrays during cell division. Mitotic microtubule array comparison, flow cytometric analysis and expression assays of cyclin-dependent kinase (CDK) complexes in root-tip cells showed that cell cycle progression is affected in bc12 mutants. BC12 is very likely being regulated by CDKA;3 based on yeast two-hybrid and microarray data. Therefore, BC12 functions as a dual-targeting kinesin protein and is implicated in cell cycle progression, cellulose microfibril deposition and wall composition in the monocot plant rice.