Abstract: It has been known that eukaryotic genomes span a wide range of sizes regardless of organism complexity. The observed differences in genome size are primarily due to polyploidy level and abundance of non-coding DNA, especially the contribution of transposable elements (TEs). Here we reviewed the current progress in genome size variation of plant species and the underlying evolutionary forces that contribute to genome expansion or contraction. Polyploidization and the accumulation of transposable element are the primary contributors to genome expansion. As to the mechanisms of DNA loss, unequal homologous recombination and illegitimate recombination are thought to be the counterbalances to the unlimited expansion of a genome. The evolutionary direction of plant genome size is also discussed, which tends to favor larger ge-nomes with deletion mechanisms acting to only attenuate genome expansion but not reverse.