Combustion instability is one of the major challenges in Lean Premixed Pre-vaporized (LPP) combustion technology. The effect of inlet conditions on combustion instability has been widely studied. But the diffuser, located between the compressor outlet and combustor inlet, has received little attention regarding its effect on combustion instability. This paper experimentally investigated the combustion instability in a single sector LPP combustor with and without a diffuser. Pressure data has been recorded and used to indicate the oscillation of the system. The results show that with the diffuser the combustor showed no prominent oscillation, and strong thermoacoustic oscillation at around 400 Hz was observed without the diffuser. Further analysis has been conducted by using acoustic net model. The resonant mode frequency of the entire combustor system and each section separately were calculated. It was found that in the case of the diffuser, the resonant frequency shifted from 400 Hz to 453 Hz, which avoiding interacting with heat release within the flame tube, thus help to stabilize the combustor system.