imfindcircles might be a good place to start

http://www.mathworks.com/help/images/ref/imfindcircles.html

Thanks John for your answer. But imfindcircle finds the already present circles in the image. I need a function which can draw a circle with single pixel line width at location and radius of own choice.

Here is a function that I wrote a few years ago for drawing circles in matlab. Although this is not quite what you are looking for, it could be modified easily. This function draws circles onto a greyscale image, adding the pixel intensity of the circle to what was there previously. This function accepts five parameters:

cy = location of centre of circle vertically within the image

cx = location of centre of circle horizontally within the image

r = radius of circle to draw

amp = increase in pixel intensity of the image caused by drawing the circle

image = 2D array containing image

    function image = DrawCircle(cy, cx, r, amp, image)

     [o p] = size(image);

     for dx=-r:r

          dy = round(abs(sqrt((r^2)-(dx^2))));

          if (((cy+dy) < o) && ((cx-dx) < p) && ((cy+dy) > 1) && ((cx-dx) > 1))

          end;

     end;

     image(round(cy+dy),round(cx-dx)) = image(round(cy+dy),round(cx-dx))+amp;

     end

Hope this helps.