During the past decades the bifurcation of limit cycles of the following planar polynomial system (x)=Pn(x,y), (y)=Qn(x,y).(1)has been considerably investigated and has now become a very popular topic in the area of applied mathematics [J.Math.Anal.Appl.305 (2005) 617], where Pn and Qn are two polynomials of degree n.It should be mentioned that the system (1) is related to the celebrated Hibert's 16th problem.