Casing drilling has been recognized as a practical method to reduce drilling costs and avoid particular drill string problems that often occur in conventional drilling processes. To gain a better understanding of the whirling motion of rotating casing, the energy method was adopted to calculate the critical whirling speeds of casing for both pinned-pinned and fixed-fixed types of support. The calculated critical whirling speed of rotating casing for fixed-fixed supports was found to be 2.35 tunes than that of pinned-pinned supports. To observe the motion patterns of rotating casing in different cases, experiments were conducted in a transparent wellbore. Rotary speed and inclination angle were found to be the two main factors that affect the motion pattern of rotating casing. For the case of 300 gallons per minute (gpm) flow rate with annular fluid of water whirling motion was observed to initiate with a rotation speed of 120 rpm. Steady whirling motion was observed at similar to 155 rpm and higher With increasing inclination angles, whirling motion was found less likely to occur The backward whirling was not observed in the experiments because the friction between casing and simulated wellbore was not large enough. The variations of torque as a function of rotary speed at different flow rates, inclination angles, and rates of penetration (ROP) were also investigated, and the results were found useful for predicting the quality of borehole cleaning. It was shown that flow rate, inclination angle, and ROP have notable influence on the torque required to rotate casing. With increasing inclination angle, torque was found to increase and reach the maximum value at the horizontal position. For a given ROP, torque was found to decrease with increase in flow rate. For a given flow rate, torque was found to increase with increased ROP.