Combustion instability is a serious obstacle for the lean premixed combustion of gas turbines and can even cause fatal damage to the combustor and the entire system. Thus, enhanced understanding of the mechanisms of combustion instability is necessary for designing and operating gas turbine combustors. In this study, in order to elucidate the instability phenomena, an experimental study was conducted in a rearward-step dump combustor with LPG and air. The fuel supply conditions and the mixing distances (L-fuel) between fuel and air are used as experimental parameters to examine the effects of fuel modulation and unmixedness. The fluctuations of pressure, heat release, and equivalence ratio were measured by a piezoelectric pressure sensor and a high speed intensified charge coupled device (ICCD) camera, respectively. The unmixedness was measured by acetone laser induced fluorescence (LIF) at nonreacting flow because of stratification of the fuel in air. Various combustion modes occurred in accordance with the equivalence ratio and the fuel supply conditions. In the case of the fully premixed condition, the spatial fuel distribution inside the combustion chamber exists in a homogeneous state compared with the partially premixed condition, which leads to instant heat release, with relatively greater intensity for the chemiluminescence of the flame and higher amplitude of pressure fluctuations. On the contrary, in the partially premixed condition, the spatial fuel distribution exists in a heterogeneous state from the combustion chamber entrance, and the flame's burnout or reignition progresses in accordance with the stratified distribution of fuel. The unmixedness of fuel and air leads to a relatively smaller intensity for the chemiluminescence of the flame and reduced amplitude of pressure fluctuations.