Electrical control of magnetic properties is crucial for device applications in the field of spintronics(1-20). Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals(9,15,17) as well as in semiconductors(6,10,11,13), the electrical control of Curie temperature has been realized only in semiconductors at low temperature(4,5,8). Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film(21,22) covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about +/- 2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.