According to recent soil survey data, it has been reported that acid sulfate soils cover areas of about 11,670,000 hectares all over the world and about 40% of those are in South-East Asia. Most of crops could not be grown on those areas without any amelioration due to low soil pH. This report consists of two parts. One is on acid sulfate soil formation in laboratory experiment. Another is on a trial of acid sulfate soil amelioration in field experiment. The results obtained on acid sulfate soil formation, in order to make clear the relationships between the process of oxidation of sulfur compounds and the moisture conditions (moisture suction) in coastal muddy soils, in laboratory experiment are summarized as follows; The concentration of sulfuric ions increase as oxidation of sulfur compounds contained in pyrite, etc. occurs, and as a consequence, the soil pH decreases in the coastal muddy soil (Fig.1). Especially, the soil pH decreased remarkably in the range of pF 2.3~3.4. The activity of soil microorganism takes part in this reaction by helping out with bringing about sulfur oxidation and formation of sulfuric ions. The pH decrease in the soil with autoclaved treatment was only slight at pF values lower than pF 3.0. Accordingly, it is concluded that the decrease of pH in coastal muddy soil occurred mainly due to soil microorganisms which oxidize sulfur compounds under the condition of higher-water than pF 3.0. The results of field experiment is summarized as follows; As a general rule, it is able to remedy the surface acidification of peat/acid sulfate soils by mixing lime dust with plough layer. However, the effect of mixed lime dust is easily disappeared in the area where the acidic ground water table is always high and moves up over the surface in rainy season. Therefore, it was carried out that several experiments for seeking protection against surface acidification due to capillary upward movement of acidic ground water by means of inserting a buffering layer of lime gravel (thickness of lime gravel layer =5㎝) between the top soil and the acidic subsoil (20 ㎝ depth) in the fields, Munoh series which is young acid sulfate soil, at the Narathiwat Land Development Center in South Thailand. The results of soil pH measurement carried out in the field in November of 1985, August of 1987 and September of 1990, it coincides with 2,4 and 7 years respectively after the beginning of the experiments, are shown in Table 3. In the plot of P2 and U1, application of lime dust to the plough layer can increase soil pH rapidly but it's residual effect is not so long. Within a 5 year period, the soil pH is likely to return to below 4. On the other hand, an underlaid lime gravel layer tends to improve the soil around the lime gravel only slight but steadily with time. Therefore, it is concluded that surface soil acidification caused by the upward movement of acidified ground water would be protected against, rapidly and continuously, by both mixing lime dust with plough layer and underlying lime gravel below plough layer. If acid sulfate soil areas would be used as upland fields, much of organic matters in soil are easily decomposed by drying. Therefore, it is suggested that it would be better for these areas to be used as paddy fields, also from the above results.