OVERVIEW OF THE STUDY Over the past decade, supply chain management has influenced businesses around the world. Purchasing is a function within the supply chain that becomes the most important element in establishing value-added products or services. The main aspect of purchasing is supplier selection. It is recognized that the selection of the appropriate supplier is one of the key success factors of a company. Selecting the right supplier is always a difficult task for the purchasing manager (Liu et al., 2005). The right selection of suppliers can lead the company to success and the achievement of its objectives. On the contrary, the wrong supplier selection can lead the company to financial problems. Therefore, purchasing departments need to emphasize supplier performance evaluation which not only covers one criterion such as price but also considers multiple criteria to evaluate suppliers in order to select the right supplier. 1.1 Background of the Study Nowadays, the oil and gas industry plays an important role in driving business both locally and globally. It provides energy and essential chemicals for industry, homes and transport that are increasingly consumed by users every day. There are around fifty oil and gas companies located in Thailand and ten out of fifty are customers of ABC Company. ABC company is a leading trading company, wholesaler, exclusive distributor and engineering company providing industrial process equipment of piping materials, such as tubing, flanges and fittings. The company was founded in May 1998 with a 100% Thai shareholder. The company provides materials and supplies to the oil and gas industry to construct and maintain transmission pipeline and shutoff projects. Figure 1.1: Piping Material of ABC Company Description Image Pipeline Flange Fitting Source: ABC Company The amount is also incessantly increased by 11.87%. The increase in purchases comes from the company's commitment to stock the material for supply at any time the customer needs to meet his satisfaction. Table 1.1 shows the comparison between sales data and purchase data from 2012 to 2013 of ABC company. Table 1.1: Sales and purchase data of ABC companyYear % Increase2012 2013Total sales (million THB) 747 780 4.42% (about 33 million THB) Total purchases (million THB) 438 490 11.87% (about 52 million of THB) Source: ABC Company During the year, ABC Company must purchase piping materials consisting of pipes, flanges and fittings according to the proportion shown in figure 1.2. Figure 1.2: Proportion of purchase of the ABC Company Source: ABC Company Source: ABC Company From figure 1.2, the amount purchased by the ABC Company from the year 2012 to 2013 of pipes, flanges and fittings is equal to approximately 68%, 18% and respectively 14%.