Micro RNAs are small, noncoding RNAs involved in the regulation of gene expression in eukaryotes. Being transcribed in the nucleus in the form of a long primary precursor, a primary microRNA undergoes multistep biogenesis that leads to the formation of a mature microRNA. One of the critical biogenesis steps is the recognition and processing of primary microRNAs by microprocessor complex. Secondary structure of a primary microRNA is considered to play a key role during this biogenesis step, but specific structural motives have not yet been identified. As a result of a study on structural and thermodynamic features of intergenic and intronic microRNAs, we have demonstrated that the two main classes of human microRNAs have no statistically significant difference in respect to these parameters. The conclusion may be a base for combining the two class microRNA sequences in a common set for the search for new microRNAs. Specific structural and thermodynamic features in human primary microRNA transcript hairpins have been found: the data may be used for search of new microRNA candidates in the human genome and for a design of artificial microRNA sequences for gene therapy and functional genomics purposes.