Heterosis is a fundamental biological phenomenon characterized by the superior performance of a hybrid over its parents in many traits, but the underlying molecular basis remains elusive. To investigate whether DNA methylation plays a role in heterosis, we compared at single base-pair resolution the DNA methylomes of Arabidopsis Ler and C24 parental lines and their reciprocal F1 hybrids that exhibited heterosis for many quantitative traits. Both hybrids displayed increased DNA methylation across their entire genomes, especially in transposable elements. Interestingly, we found that increased methylation of the hybrid genomes predominantly occurred in regions that were differentially methylated in the two parents and covered by small RNAs (sRNAs), implying that the RNA-directed DNA methylation (RdDM) pathway may direct DNA methylation in hybrids. In addition, we found that 77 genes sensitive to remodeling of DNA methylation were transcriptionally repressed in both reciprocal hybrids, including genes involved in flavonoid biosynthesis and two circadian oscillator genes, CIRCADIAN CLOCK ASSOCIATED1 and LATE ELONGATED HYPOCOTYL. Moreover, growth vigor of F1 hybrids was compromised by treatment with an agent that demethylates DNA, and by abolishing production of functional small RNAs due to mutations in Arabidopsis RNA methyltransferase HUA ENHANCER1. Together, our data suggest that genome-wide remodeling of DNA methylation directed by the RdDM pathway may play a role in hybrid vigor. Examination of small RNA sequencing in 2 Arabidopsis ecotypes and their reciprocal hybrids.