Thai investment abroad has continued to grow and will play an increasingly important role in the economy as new engines that will help propel the country in the next phase. Foreign investment is another important tool for Thai businesses to expand their business, including an engine in uplift the country's economy. The research study focuses on Gems or Jewelry as a product for overseas investment in overseas. The problems may stem from two reasons: difficulty of increasing market share in Thailand, and insufficiency of investment budget to seek new foreign customers elsewhere. The research on the factors of investment decision making affecting the Market Share Expansion in Belgium had the objectives shown as follows: 1) to study the investment decision making affecting the market share expansion in Belgium, 2) to analyze the relationship between the investment decision making and the market share expansion in Belgium, and 3) to analyze the content analysis with a qualitative method of the investment decision making and the market share expansion of the Belgium. The researcher collected data using an in-depth interview of Thai businesses of Gems and Jewelry and an observation report (secondary data) of the office of government responsible for export purpose. The institutions were categorized by Gemstone and Jewelry business structure. This research study also examines the SWOT analysis and proposes the business strategy in allowing Thai entrepreneurs who can be beneficial for the findings used for their future operations The results showed that the gem and jewelry business in Belgium can be expanded by setting-up a representative company from Thailand in order to support export business by increasing marketing channel and market share expansion. It is clear that Belgium or Europe incur high costs to operate with highly skilled workers. Several factors the entrepreneurs considering are resources, promotion, or market that may have an impact for them to making any decision to invest money in overseas for business ...