We study the single particle and collective dynamics of a storage ring where the momentum compaction is made small to make the ring nearly isochronous and reduce the bunch length and increase the peak current. We find that a quasi‐isochronous ring makes it possible to obtain a bunch length in the millimeter range, about one order of magnitude shorter than the present values, and to increase the beam longitudinal brilliance by more than one order of magnitude. The shorter bunch length allows one to reduce the betatron function and the beam area at the interaction point of an electron‐positron collider, leading to larger luminosity. We also design a ring whose momentum compaction can be varied over several orders of magnitude.