EllipticArcClass ellipticalArc = new EllipticArcClass();

IEnvelope env = new EnvelopeClass();

IPoint lowerLeft = new PointClass();

lowerLeft.X = centerPoint.X - width;

lowerLeft.Y = centerPoint.Y - height;

IPoint upperRight = new PointClass();

upperRight.X = centerPoint.X + width;

upperRight.Y = centerPoint.Y + height;

env.LowerLeft = lowerLeft;

env.UpperRight = upperRight;

ellipticalArc.ConstructEnvelope(env);

IGraphicTrackerSymbol graphicTrackerSymbol = this.graphicTracker.CreateSymbol(simpleFillSymbol, null);

int shapeId = this.graphicTracker.Add(ellipticalArc, graphicTrackerSymbol);

http://gis.stackexchange.com/questions/5228/how-to-create-a-circle-with-arcengine

How to create a circle with ArcEngine?

C#