In this paper we find necessary and sufficient conditions in order that a planar quasi-homogeneous polynomial differential system has a polynomial or a rational first integral. We also prove that any planar quasihomogeneous polynomial differential system can be transformed into a differential system of the formu = uf (v),v = g(v) with f (v) and g(v) polynomials, and vice versa.2010 Mathematics Subject Classification. Primary: 34C05, 34A34, 34C20. Key words and phrases. Quasi-homogeneous polynomial differential equations, integrability problem, polynomial first integral, rational first integral.