Evaluation of source or sink limitations on maize (Zea mays L.) yield and yield components is important for the rational design of agricultural practices as well as breeding strategies. There is little information on how the source or sink limitations during the effective grain-filling period affect final kernel quality. The objective of this study was to evaluate the effect of source/sink manipulation in different times after mid-silking stage on kernel weight (KW), kernel number per square meter (KN m^-2), kernel protein and oil of different maize hybrids. Field experiments were conducted at the experimental farm, College of Agriculture, Shiraz University, Shiraz, Iran, located at Bajgah (52° 35x N, 39° 4x E, and 1810 m above sea level) during the 2008 and 2009 growing seasons. The treatments included three hybrids and defoliation that were arranged in the main, and subplots, respectively. Leaves removal treatments were imposed when plants were at the mid-silking stage. Silking was taken as the time when 50% of the plants in a row presented visible silks. Defoliations included control, 50% of defoliation at 25 (early defoliation), and 35 (late defoliation) days after silking (defoliation treatments were applied to all plants in each plot). The experiments were conducted in RCB design with three replications and the treatments in a split-plot arrangement. Early defoliation greatly reduced kernel growth rate and grain filling period, resulting in lower mean kernel weight. Defoliation treatments modified KN m^-2 and kernel number per ear (KNPE) and variations in these components affected protein and oil content. Decreased source size by defoliation decreased kernel protein content drastically with no significant change in oil content. Differences among treatments appeared to be related only to the post-flowering source/sink ratio. Therefore, to improve protein yield in maize, hybrids and agronomic practices should aim to increase the post-flowering source/sink ratio.