We propose a way of synthesizing Thai vowels, monophthong and diphthong which consist of nine short-long vowel pairs and three short-long vowel pairs respectively. By adjusting the fundamental frequency or pitch contour, the five tonal of these vowels can be synthesized, leading to a total of 120 phonemes. We evaluate the synthesized monophthongs by comparing the first and second formants to standard in terms of different percentages, which result in 9.89% and 7.44% respectively. For the synthesis of diphthongs, we slide from one vocal-tract vowel to another using linear, logarithmic, or exponential functions.