If concrete is observed at the microscopic level, it can be seen to contain many interfacial transition zones (ITZ), resulting in the weakening of its mechanical properties; as the physical properties of lightweight aggregates (LWA), such as their high absorption, are clearly different from those of normal weight aggregates (NWA), they may lead to variations in the ITZ of lightweight aggregates concrete (LWAC), making its mechanical behavior different from that of normal concrete.This study takes three types of LWA with different rates of absorption as its subjects in order to examine the effects of LWA absorbency on ITZ. The main variables tested include water/binder (W/B) ratio, amount of fly ash substituted for cement, and the saturation states of the four types of LWA, with three different types of microscopic tests used to observe the microstructure of ITZ in concrete, and analyze and compare their differences with ITZ in concrete made with NWA. The results of the study indicate that in LWAC, ITZ with weaker tensile strength did not appear around highly absorbent LWA; only the sample group with a W/B ratio of 0.29 did not show apparent absorption due to the higher viscosity of the mortar, resulting in a slight downward trend in tensile strength, but it was still superior to the tensile strength of ITZ in typical NWA.Observation using scanning electron microscopy (SEM) found that inner pores and cracks of ITZ in LWAC were all smaller than in NWC; X-ray diffraction (XRD) tests indicated that the amount of CH crystals in ITZ was greater than LWAC; and microhardness testing found that microhardness values were higher closer to the surface of the LWA, with some even exceeding that of concrete. The increase in the tensile strength of ITZ in LWAC subsequent to the addition of pozzolanic materials was limited; their improvement of the weakness planes formed by ITZ in NWA concrete, however, was more apparent. An LWA has higher water absorbency capacity when in an absolutely-dry condition, which can markedly increase the strength of ITZ; conversely, if an LWA is in saturated surface dry (SSD) condition, its ITZ behave like those of concrete.