Target mimicry is a recently identified regulatory mechanism for microRNA functions in plants in which the decoy RNAs bind to microRNAs via sequence complementary therefore to block the interaction between microRNAs and their authentic targets. Both endogenous decoy RNAs (microRNA target mimics) and engineered artificial RNAs can induce target mimicry effects. Yet till now, only the IPS1 (Induced by Phosphate Starvation 1) RNA has been proven to be functional endogenous microRNA target mimics (eTMs). In this work, we developed a computational method and systematically identified intergenic or non-coding gene-originated eTMs for 20 conserved microRNAs in Arabidopsis and rice. The predicted microRNA binding sites were well conserved among eTMs of the same microRNA, whereas sequences outside of the binding sites varied a lot. We proved the eTMs of miR160 and miR166 as functional target mimics and identified their roles in the regulation of plant development. The effectiveness of eTMs for other three miRNAs was also confirmed by transient agroinfiltration assay.