If you have candidate miRNAs and want to determine conservation in mice and humans, perhaps the easiest and quickest is type in the sequence in the mirbase.org search.

Another tool which might help is miRviewer: http://people.csail.mit.edu/akiezun/microRNAviewer/index.html

Please visit miRBase.

Thanks. Actually I know that miRBase contains all annotated miRNAs. My is question is how to show the conservation between huge number of miRNAs between species. So simply I wanted to see how conservative is all miRNAs of human to those in chicken. 

Name is very informative. hsa-miR-XXX is the orthologus of gga-miR-XXX. miRBase also provides the miRNA family annotations. If you want to know detailed conservations, please visit UCSC genome browser with the target miRNA regions.