Whole-genome duplication produces massive duplicated blocks in plant genomes. Sharing appreciable sequence similarity, duplicated blocks may have been affected by illegitimate recombination. However, large-scale evaluation of illegitimate recombination in plant genomes has not been possible previously. Here, based on comparative and phylogenetic analysis of the sequenced genomes of rice and sorghum, we report evidence of extensive and long-lasting recombination between duplicated blocks. We estimated that at least 5.5% and 4.1% of rice and sorghum duplicated genes have been affected by nonreciprocal recombination (gene conversion) over nearly their full length after rice-sorghum divergence, while even more (8.7% and 8.1%, respectively) have been converted over portions of their length. We found that conversion occurs in higher frequency toward the terminal regions of chromosomes, and expression patterns of converted genes are more positively correlated than nonconverted ones. Though converted paralogs are more similar to one another than nonconverted ones, elevated nucleotide differences between rice-sorghum orthologs indicates that they have evolved at a faster rate, implying that recombination acts as an accelerating, rather than a conservative, element. The converted genes show no change in selection pressure. We also found no evidence that conversion contributed to guanine-cytosine (GC) content elevation.