[en] The effects of thermal processing and alloying elements on the microstructure and phase transformation of N18 and N36 zirconium alloys were investigated using Optical Microscopy (OM), Transmission Electron Microscopy (TEM), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). The results showed that the difference of alloying element niobium among the two alloys was significant, and the α → (α + β) phase transformation temperature decreases with an increase of niobium content. Niobium is an element extending β phase region. T_α→α+β of N18 and N36 measured by DTA and DSC were higher than those of the results from TEM. It was shown that the variation of absorbing heat could be recorded with possessing certain β-Zr fraction at heating and it maybe related to the rate of heating when measured by the thermal analysis methods. Hydrogen in the alloys extends the β phase region and accelerates the nucleation and growth of β-Zr in the process of heating. The hydrogen was also mainly soluble in the β phase of α + β temperature region and resulted in a different microstructure. A consequence of hydrogen ingress into Zr alloy is not only to affect its mechanical behavior, but also decrease the α → (α + β) phase transformation temperature