Two-phase flow instability in a system including downcomers is studied experimentally. The system used for experiments consists of two parallel channels each composed of a “first riser”, a downcomer and a “second riser” in order. Four series of experiments are carried out with a first-riser-heated single channel system, a downcomer-heated single channel system, a first-riser-heated parallel channel system and a downcomer-heated parallel channel system. A dynamic instability is observed in the experiments with the single channel system, while a kind of static instability, “one-sided flow” as well as a dynamic instability, are observed in the experiments with the parallel channel system. As it is proved that the static instability, in this case, results from the negative slope in W-ΔP characteristics which is mainly due to the term of static pressure loss in the downcomer, it is concluded that this instability is peculiar to systems including downcomers. Another instability associated with condensation of v...