The whole country area belongs to the tropical climate. The tropical rain forest occupies 80% of the national land (4,690 km2). Seventy per cent of the forest is virgin, and half of which is environmentally preserved. The land is roughly divided into eastern and western regions, and the eastern region, Templon River basin, is undeveloped forest except seaside, and forms a vast national park. Most of the population lives in the three district in the western region.

Agricultural productivity of Eastern Asia countries including Brunei is low compared to the monsoon region. Tropic soil easy loses nutrition salts, and is not suitable for agriculture. Leaves and cut trees are soon decomposed by microbes and termites, leaving no humus behind. Additionally, as the effect of heat and water, soil components other than aluminum oxide and iron oxide are easily washed away, and the soil is barren. In the rain forest, the nutrition to support the forest is collected not by the soil, but by the trees and plants at the canopy.

Small scale agriculture includes dry rice cropping in the forest in the mountain region and rice cropping at the terraced paddy field. For short term, taros are produced by slush and burn agriculture to use the nutrition collected by the forest biomass, but this destroys the forest, which is the main nutrition collector, and the soil nutrition is used up in 2 years, and after that the land becomes barren.

At the low wet land where the washed nutrition is stored, Sago palm plantation is possible. Swamp forest arises at this kind of land. This swamp forest is composed of trees with low height, and the plants grows only by single layer. This leads to good supply of light, but since the oxygen in soil lacks, humus decomposition is prevented, and peat is formed. Thus, even if agricultural field is made, the surface begins to sink soon to form pad. Agriculture is difficult in this region.

At the seaside exists mangrove forest at the brackish water region where seawater is comes and goes with low and high tide. The soil of this forest is strongly acidic due to the root acid that mangrove root secrets. This land cannot be used for slush and burn agriculture, and was barren in terms of agriculture. It was not until 20th century that it got used for fish breeding.