Nine conserved miRNA families and three potential novel miRNAs in Brassica rapa were identified from a small RNA library. The expression patterns of some conserved miRNAs had different tissue specificity in Brassica and Arabidopsis. One of the three potential miRNAs, named bra-miR1885, was verified as a true functional miRNA. It could be induced specifically by Turnip mosaic virus (TuMV) infection, and target TIR-NBS-LRR class disease-resistant transcripts for cleavage. Based on the hypothesis for de novo generation of new miRNA genes and the sequence similarity between bra-MIR1885 precursor loci and target transcript sequences, we suggest that bra-MIR1885 is a new miRNA gene that originated through inverted duplication events from TIR-NBS-LRR class disease-resistant protein-coding gene sequences, which became bra-miR1885 targets.