The first 26R channel structure, NTHU-5, built up with two types of helical metal phosphite chains has been synthesized under mild hydrothermal conditions; this novel open framework is composed of both octahedral Al3+ and tetrahedral Zn2+ centers and represents the first example with participation of aluminum in a pore size over 20R; the 26R channel is truly a stack of 26-membered rings instead of just an opening; this study has successfully demonstrated that two differently charged metal ions can form M-O-P-O-M' bonded structure with ring size larger than any other existing inorganic open frameworks; in addition, varied metal analogues can be prepared using the same approach; the system of NTHU-5 has provided an intriguingly new chemical synthesis for inorganic microporous materials with interesting reaction conditions.