The low flammability of poly(3,3'-dihydroxybiphenylisophthalamide) (PHA) and its halogen, methoxy, phosphinate, or phosphate derivatives was characterized by multiple analytical techniques, emphasizing the thermal decomposition behavior is the basis for the polymer flammability. Pyrolysis gases from these polymers were identified by pyrolysis GC/MS. The residual chars were characterized by elemental analysis. Simultaneous thermal analysis was applied to study the thermal decomposition process, and flammability was measured by a milligram-scale pyrolysis-combustion flow calorimeter (PCFC). It has been found that PHA and its halogen derivatives have extremely low flammability. They all exhibit a two-stage thermal decomposition process, where the first stage corresponds to cyclization. into a quasi-poly(benzoxazole) (PBO) structure and the second stage is the random scission of the PBO backbone. However, the methoxy, phosphinate, and phosphate derivatives exhibit very different behaviors in thermal decomposition and flammability. Most of the PHAs produce a large amount of char, around 40-50%,. Thermal decomposition mechanisms of PHAs were also proposed.