In this paper, we propose a spherical mobile robot equipped with eccentric twin rotors and 2-d.o.f. actuators, which we call Volvot. The first actuator, called rotor actuator, drives a pair of off-centered metal disks fixed to both ends of the drive shaft. The second actuator, called base actuator, rotates the whole internal block relative to the spherical outer shell. An advantage of adopting eccentric rotors is that they are able to make use of two types driving force for locomotion; one is the gravity force due to uneven mass balance, and the other one is the reaction torque proportional to the angular acceleration of the rotors. In this paper, we derive its mathematical dynamic model of the spherical robot under rolling contact constraints, considering its rotation about the vertical axis which had been ignored in most of the past literatures. We then propose a periodic control scheme to perform steady locomotion based on the principle of holonomy, followed by some numerical simulations and experiments to examine the proposed approach.