Sometimes while you modeling parts it could happen that the circle looks like a polygon. That is happening because of predefined parameters in the Display Performance Options. This anomaly is due to the fact that CATIA must work properly with performances which are good enough for work on stronger and weaker workstations.
To improve display performance, you need to change parameters in Tools / Options / General / Display, then choose the Performance tab. Here you will find two parameters which can be modified for 3D Accuracy and 2D Accuracy.
As you can see on the picture, with a fixed value of 0.6 for 3D Accuracy, the circle is displayed as a polygon. If you reduce the value, the display will become better. 3D Accuracy refers to 3D modeling, while 2D Accuracy refers to Drafting module, and both improve Display performance.
3D / 2D Accuracy Proportional calculates the accuracy of the display according to the size of the object. The bigger size of the object means rougher display. For the same value, the display will always be finer for smaller objects than for bigger ones.
3D / 2D Accuracy Fixed uses the same value to calculate the display on all objects, which means that it does not depend on the size of objects.
If the performances of the workstation are on the low level while you are working with more complex objects, it is necessary to increase the value for 3D Accuracy until you achieve the balance between visual and hardware performances.