When rotation of an impeller is started, the torque is larger than that at a steady state. This torque is important for the design of the impeller. However, the relationship between the starting torque and the rotational speed and the shape of the impeller has not yet been sufficiently investigated. The present study investigates the influence of the rotational speed, the number of blades, and the blade width on the starting torque of a vertical paddle impeller. Furthermore, experiments and a CDF study were conducted to examine the mechanism generating the starting torque. The starting torque was separated into the 1st and 2nd starting torque. The 1st starting torque was generated while the impeller was accelerating and CFD results showed that the flow was only around the impeller blades. It was found that the 1st starting torque correlated with the angular acceleration of the impeller and the fluid density. The 2nd starting torque was generated after the impeller reached a steady rotational speed. In this period, the negative pressure region at the back of the blades increased. It was also found that the 2nd starting torque originated from the profile drag of the blades and was proportional to the square of the impeller rotational speed and the blade width. Also, it was proportional to the 0.6th power of the number of blades. In the present study, the relationship between the 2nd starting torque and the torque under completely baffled conditions was also investigated, and it was confirmed that they were almost equal. From this result, it was found that the measurement of starting torque was a possible method for obtaining the torque under completely baffled conditions.