Jointed composite structures (JCSs) are often used in the marine, automotive and civil engineering industries. In JCS, thin carbon-fiber-reinforced composite laminates are bonded with epoxy adhesives. But, disbonds can occur at the bondinterphase due to variable environmental conditions, cyclic loading, aging, fatigue, amongst others, which may lead to a substantial reduction in load-bearing capacity of the structural assembly. Hence, it is essential to identify these hidden disbonds, and the identification becomes more challenging due to frequent change in ambient temperatures. It is found that the ultrasonic guided wave propagation based inspection technique is suitable for inspection of such complex multilayered structures. The aim of this paper is to investigate the disbond effects on the propagating wave modes in the JCS under variable ambient temperatures. Towards this, a series of finite element based numerical simulation of guided Lamb wave propagation in JCS under variable temperature is carried out in ABAQUS using piezoelectric actuator-sensor transducers. Laboratory experiments are then conducted to investigate the disbond effects and a good agreement is found between the simulation and experimental results.