Grain filling, the accumulation of assimilates in grain, determines the final grain weight, a major component of grain yield in wheat. The assimilate for grain filling is originated from current photosynthesis and/or remobilized culm reserves. Culm reserves play a vital role in grain filling when current photosynthesis is restricted owing to senescence. Most of the wheat cultivars grown in Western Japan experiencing much precipitation show poor grain filling resulting in smaller grains and finally in lower grain yield. The mechanisms and causes of such poor grain filling were studied. The poor grain filling in western Japanese wheat cultivars is attributable to the limited current assimilation and poor remobilization of culm reserves to the grains. The waterlogging conditions in the field might be one of the important causes of poor grain filling. These findings improve our understanding of grain filling in wheat and other cereals, and will be useful to Agronomist, Crop Stress Physiologist or anyone else who are interested in Grain Filling in Cereals.