In this paper we consider differential systems of the form \begin{equation}\label{equ:(1)} \begin{array}{l} \left\{ \begin{array}{l} \frac{{{\rm{d}}x}}{{{\rm{d}}t}} = y + {P_2} + \cdots + {P_n}\\ \frac{{{\rm{d}}y}}{{{\rm{d}}t}} = - x + {Q_2} + \cdots + {Q_n} \end{array} \right. \end{array} \end{equation} with${P_i}$and${Q_i}$homogeneous functions of degree$i$.Suppose that$f = \sum\limits_{k = s}^r {{f_k}} = 0$is an invariant algebraic curve of (\ref{equ:(1)})and${L_f}$is cofactor of$f$. We show that${f_s} = {\left( {{x^2} + {y^2}} \right)^m}$with$s = 2m \ge 0$and${L_f}(0,0) = 0$. We also present a method to compute the exponential factor of differential systems. These results are used to determine the type of equilibrium point. \Keywords{Invariant algebraic curve, Cofactor, Exponential factor, Equilibrium point.}