According to David Fontana, the most basic of all mandalas is the circle, an intriguing symbol that has no beginning and no end. It is the still centre of the turning world, the magic circle that defines and protects a sacred space within which one finds tranquillity and peace. The circle of mandala is a primal symbol for all that exists. It is the symbol of the sun, the giver of light, and of the full moon, the discloser of dark mysteries, the wheel of life, the symbol of ultimate perfection, the tunnel between this world and the world to come, the symbol of eternity. Although the circle may contain other shape within it, such as the square and the triangle, and sometimes may even be bounded on the outside by one or other of these shapes, the circle remains the primary feature of all mandalas. Without the circle, there is no mandala. But once the circle is drawn, then other symbols can be added to it. (Source: Meditating With Mandalas: 52 New Mandalas to Help You Grow in Peace and Awareness)
The symbolic meaning of circle in a mandala is completion, movement, perfection, with no end and no beginning.
The pattern of a mandala -- a circle with a center -- reaches far beyond a two or three dimensional art form. Examples of mandalas are found in nature. Every cell in our body, for instance, is a living mandala. So is the iris of our eye, a snow crystal, a bird's nest -- even a bicycle wheel. Look around. Can you see more mandala in your surroundings?