As far as I am aware there are no known examples of circular (ring) chromosomes being normal in eukaryotes. Large ring chromosomes usually lead to further instability because of mitotic crossing over resulting in dicentric or nulli-centric chromosomes and to breakage-fusion cycles involving further losses or gains of DNA and centromeres. In humans very small ring chromosomes can be maintained for long periods both in vivo and in vitro because the frequency of mitotic crossing over is low and the greater likely-hood that such small chromosomes do not contain contain genetically important chromosome regions.

Thank you very much Peter L Pearson for your nice explanation.