MicroRNAs are derived from endogenous stem-loop precursors, and play important roles in various biological processes. From next-generation sequencing data, it is suggested that both the 5p-arm and 3p-arm of mature miRNAs could be generated from a single miRNA hairpin precursor; however, the current miRNA databases fail to provide comprehensive arm annotation features, which could result in ambiguous and incomplete analyses. In the present report, we have annotated over 99.7% of miRNAs with the correct 5p-arm and 3p-arm features. The length distribution of all annotated miRNAs is around 22 nucleotides; however, the 5p-arm miRNAs seem to be longer than those of the 3p-arm, which is evident in the 23-nucleotide group. Our study effort generates comprehensive miRNA arm-feature annotation which can be utilized for better interrogation of miRNAs. In further analysis of human gastric cancer tissues, we identified 38 down-regulated miRNAs and 22 up-regulated arm-specific miRNAs using this new comprehensives miRNA list.