Often plotting or conversion packages do not use high- resolution output devices efficiently. Where such packages use very large point file to describe a plot. In this thesis, we will propose a new method which produces a conic spline to approximate the parametric function which could be represented by R(t) = [ X(t),Y(t) ]. We choose the knot in the inflection point which makes the splited segments as less as possible. And we select the conic spline as the primitive curve, because it's simple, fast in computing time, and flexible. The most important problem in this thesis is to find the accurate value of cusps and inflection points to be the knots. We'll discuss that in the approximte conversion steps.