Smoldering is the dominant mechanism of combustion in peat fires. It is represented by a slow burning, low temperature, and flameless types of combustion. The rate of spread of smoldering peat is generally considered as the main feature in relation to a rating of fire hazards. It is determined by the oxygen supply, the amount of heat generated and the amount of heat loss that is released into the environment. Since the heat balance of a smoldering combustion process is affected by the amount of heat generation and heat loss to the environment, therefore the size of the peat layer plays a great role. This study aims to observe the impact of the layer thickness on the smoldering spread of peat samples from Papua, Indonesia. The experimental works were carried out in an adjustable reactor equipped with insulation board to minimize the heat loss to the environment, a set of thermocouples spread evenly throughout the reactor, a data acquisition system, and a FLIR One infrared camera. The dry peat samples (MC 3-5 % wet base) was ignited using an electric coil heater which was powered by 100 watts of electricity, and the spread of the heat is observed using a thermocouple circuit and the infrared camera. Experiments were carried on thin peat layers of 3 cm and 4 cm. The results show that the average smoldering spread for 3 cm and 4 cm depth are 4.10 cm/h and 5.01 cm/h respectively. Higher peak temperature was observed for 4 cm depth than the 3 cm depth.