Abstract: MicroRNAs are RNAs of 21-22 nucleotides in length that are known as negative regulators of gene expression. By its molecular mode of action, have been described important biological roles in the development and physiology of plants, animals and fungi. In the model plant Arabidopsis thaliana, the miRNAs perform essential functions such as juvenile-adult transition, morphogenesis of leaves and response to biotic and abiotic factors. At genomic structure, sequencing studies demonstrate that miRNA genes are similar to genes encoding proteins. Thus, the MIR genes have promoters directing their expression in specific tissues. In this work, we cloned and characterized the promoter activity of miRNA858 in Arabidopsis. The transgenic lines generated show that miRNA858 promoter is active mainly in flower, this expression activity suggest that miRNA858 has functions in this tissue. This result is a first approximation for to know the biological role of miRNA858 in Arabidopsis thaliana.