Anti-miRNA oligonucleotides (anti-miRs) effectively and specifically inhibit the function of individual miRNAs and have the potential to serve as a novel class of nucleic acid therapeutic. However, the details of the mechanisms of anti-miRs in cells have not yet been clarified sufficiently. In particular, the localization of the complexes of anti-miRs and target miRNA in cells remains unclear. We previously developed anti-miRs composed of serinol nucleic acid (SNA) that very effectively inhibited miRNA-mediated silencing activity. Here we describe an imaging system based on the fluorescence resonance energy transfer (FRET) designed by miRNAs labeled with fluorophore-quencher pairs and an SNA-based anti-miR labeled with an acceptor dye. We discovered that the anti-miR hybridizes with the miRNA in the miRNA-induced silencing complex (miRISC), which is the active complex formed by miRNA and Ago2 in cells within P-bodies. Based on FRET ratio analysis, we hypothesize that the complex formed by the anti-miR and the miRNA in P-bodies is dynamic, with anti-miR complexing the miRISC, followed by miRNA release and degradation. Our findings provide valuable insights into the mechanism of action of anti-miRs and enable further studies of miRNA-targeted therapeutics.