On the basis of size of heterocyclic rings, classification of these compounds is done into following types:

Five-Membered Heterocyclic Compounds: These are five membered ring compounds consisting of one hetero atom and four carbon atom.

Six-Membered Heterocyclic Compounds: These compounds contain six atom ring in which five atoms are carbon while one is hetero atom.

Seven-Membered Heterocyclic Compounds: These are seven membered ring compounds consisting of one hetero atom and six carbon atom.

Condensed Heterocyclic Compounds: These types of compounds, comprises of a benzene ring fused with the heterocyclic ring containing 5 or 6 atoms.