A multiple-output DC-DC power converter using cross-regulation enables a reduction in size, weight and cost. However, the effect of the smoothing capacitances on the power converter stability has not been discussed enough. Besides, the input circuit is usually required to be isolated from the output side, and the combination of a photo-coupler and a shunt-regulator is often used. However, these components have phase delay and deteriorate the power converter stability. This paper analyzes the stability of three kinds of multiple-output DC-DC power converters using the cross regulation. In the analysis, the transfer function of the photo-coupler and shunt-regulator is estimated from the measured data. Finally, the relationship between the stability boundaries of the power converters and the smoothing capacitances is focused for the size reduction.