I think you mean in an isometric view.
Regardless, do you mean a portion of the model needs to appear circular when view in isometric?
Or do you just want a circle on the isometric view?
In in the model, then you need to create a plane (or have an existing surface) that's parallel to the isometric view (Ctrl+7) and sketch a circle.
On the drawing view, select it and create a new sketch and draw a circle.
If, for some mysterious reason, you are actually creating a 2D isometric drawing (as opposed to a 3D model), you will need to use the ellipse tool oriented to suit the isometric view.