This paper presents a rotation speed estimation and an indirect speed control method for a turbine-generator in a grid-connected 3-phase electrical power conversion system of an organic Rankine cycle (ORC) generation system. In addition to the general configuration mechanism and control techniques that are required in the grid-connected ORC power generation system, the indirect speed control method using the grid-side electric power control and the speed estimation method is proposed for the proper speed control of turbine-generators. The speed estimation method utilizes a digital phase-locked loop (PLL) method that uses a state observer to detect the positive-sequence voltages. A 10 kW system where a Motor-Generator set is used as a turbine simulator and a 23 kW actual system for the grid-connected ORC power generation were designed and manufactured, respectively. This paper includes various experimental results obtained from field tests conducted on actual installed ORC systems.