This paper discusses LFC (load frequency control) with HVDC (High Voltage DC transmission system). So far, AGC (Automatic generation control) has focused on economic dispatch control and load frequency control; especially the latter is mainly on frequency stabilization for ac-link network systems. However, the upcoming power-electronics based HVDC transmission system offers new aspects for the improvement of frequency control. In this paper, 2-area and 3-area network systems are investigated to discuss how a HVDC interconnected system can works to suppress frequency fluctuation for random load disturbance. Because DC-interconnection provides an adequate power exchange, reduction of frequency deviations for both systems is achieved if the control gain is tuned properly. However, the effect varies depending upon geometrical differences. In order to discuss the effectiveness of HVDC link for load frequency control, the HVDC-link is designed as a feedback system of system state variables, whose concept has been used for Hokkaido-Honshu link in Japan and also proposed for Minami-Fukumitsus link in Japan. The load fluctuation model is constructed to demonstrate actual load's behavior. In AC-interconnected power systems, the HVDC-link connecting the local systems does not provide frequency control enhancement since power exchange has been already achieved on the other links. However, if two isolated power systems are interconnected, the HVDC-link will make an important role in sustaining the frequency within regular tolerance even if the its capacity is very small compared to those of two systems. Whole DC-interconnected system now acts as a single system and the disturbance to even the weaker system is easily suppressed since it is treated as a tiny disturbance to the whole system.