The instabilities observed in a parallel-channel system carrying boiling fluid in forced upward flow have been studied theoretically and experimentally, using water as test fluid. The system, where the thermo-hydraulic flow conditions in two channels are different from each other, is studied theoretically and experimentally, in order to make clear the effects of the different conditions on the flow instabilities. The different conditions between parallel channels are made artificially by changing the entrance throttlings, the heater lengthes and the heat fluxes. Consequently, the instability in the system where the own characteristic frequencies are approximately equal in two channels almost agree with the one obtained under the uniform condition equivalent to the average operating condition in two channels, such as the different entrance throttlings and the different heater lengthes. On the other hand, the system, where the characteristic frequencies differ from two channels, becomes more stable with increasing the difference of flow condition, such as the different heat fluxes.