Dear Ying, A simple way to obtain what you want is the following: 1) Select an x-y mesh where you want to plot the function, using xs<-seq(xmin,xmax,length.xs) ys<-seq(xmin,xmax,length.ys) 2) Define f as a function of (x,y) and compute the values of f over the x-y mesh, by using zs<-outer(xs,ys,FUN=f) 3) Draw a contour plot at level 0: contours(xs,ys,zs,levels=0,drawpoints=FALSE) Hope it helped. [[alternative HTML version deleted]]