This thesis was about export growth, quantity of exports, market expansion andcompetitiveness of two principal agricultural products of Thailand, namely, rice andrubber, during the period 1986 to 1995. Also, the study included the relationshipbetween export instability and export competitiveness. The study used the ConstantMarket Share Model (CMS) to assess the export performance and measured the exportinstability indexes using the concept of deviation from the exponential least squarestrend line. Dividing the study period into two sub-periods, i.e. 1986-1990 and 1991-1995,it was found that, the export performance, for both rice and rubber, were higher. TheThai rice and rubber exports, in general, were still depending on traditional marketswhich expanded at a low rate, while the competitiveness of Thai rice and rubberexports declined. Even that there was a similarity between rice and rubber export growth duringthese two periods, the export instability indexes indicated some differences, i.e.with some degree of export instability, the rice exports became more stable in thelater period (1991-1995), while rubber exports became less stable. The rice marketbecame more stable with export expansion and small growth in principal markets aswell as smaller degree of competitiveness, in comparison with other competitors suchas United States and Vietnam. It was found that the demand factor was a dominantdetermining factor during the first period while the supply factor was a dominantduring the second period. The Thai rice exports was becoming more stable with changesin determining factor, the reason for instability was mainly from the supply side. In the case of rubber exports, the instability of exports became higher in thesecond period. When we consider instability with the export performance, it was foundthat the instability originated from both demand and supply sides. Therefore, weconcluded that Thailand was still having some competitiveness and instability problemeven with the fact that she was the largest supplier in the world market.