The hybrid high voltage direct current (HVDC) transmission system consists of the line-commutated converter based HVDC (LCC-HVDC) transmission system and the voltage source converter based HVDC (VSC-HVDC) transmission system, which have different fault characteristics than conventional HVDC transmission system. To solve this problem, the DC side fault transient current characteristics in hybrid DC transmission systems are investigated. Firstly, a two terminal hybrid HVDC transmission system with LCC converter station in the sending-end grid and VSC converter station in the receiving-end grid is established. The equivalent circuit in case of DC side fault is derived by using laplace transform theorem, and the simple expressions of DC fault current on LCC side and VSC side are analyzed. Secondly, on the basis of the simple expressions, the accurate fault current expressions on both sides are further analyzed by fully considering the dynamic change process of the trigger angle of the LCC converter station at the sender side and the feed-in of the AC current from the VSC converter station at the receiver side. Then, the variation characteristics of DC side fault current in three HVDC systems are comparatively analyzed in terms of fault current amplitude and harmonic. Finally, the correctness of the proposed analytical expression of DC side fault current is verified by MATLAB/Simulink simulation.