Ontology is commonly used as a structure capturing knowledge about a certain area via providing relevant concepts and relations between them. Nowadays, because of the increase in designing ontologies in different domains, it is important to describe some criteria for selecting the most appropriate ontology. The purpose of this paper is to discuss the ontology evaluation criteria, approaches and layers. At first, different evaluation stages in ontology evaluation were explained. Then the most important approaches to ontology evaluation were described: included gold standard, task-based, data-driven and criteria based evaluation. Another part of this paper was about ontology evaluation criteria such as those mentioned by experts of ontology domain as well as the criteria that proposed by US National Center for ontological Research. Lastly, the levels of ontology evaluation, involved lexical, vocabulary, or data layer, hierarchy or taxonomy layer, other semantic relations layer, context or application level, syntactic level, structure, architecture and design layer were characterized.