Four Thai synthetic chicken lines (Kaen Thong, Khai Mook Esarn, Soi Nin, and Soi Pet) originated from Thai native and exotic commercial chickens were evaluated for their growth and carcass traits with the purpose of developing a Thai broiler breeding program. Insulin-like growth factor I (IGF-I) gene is known to play an important role in growth, proliferation and differentiation. Consequently, we investigated the possibility of using the IGF-I gene for marker-assisted selection in Thai synthetic chickens. We looked for variations in the IGF-I gene and studied their association with growth and carcass traits; 1046 chickens were genotyped using PCR-RFLP methods. A general linear model was used to analyze associations of the IGF-I polymorphism with growth and carcass traits. Kaen Thong, Khai Mook Esarn, and Soi Nin chickens were found to carry similar frequencies of alleles A and C (0.40-0.60), while Soi Pet chickens had high frequencies of allele C (0.75). The IGF-I gene was significantly associated with some growth traits (body weight at hatching, and at 4, 8, 12, and 14 weeks of age; average daily gain during 0-12 and 0-14 weeks of age) in all synthetic chickens. Carcass traits (the percentage of dressing and pectoralis major) were significantly different only in Khai Mook Esarn chickens. We conclude that IGF-I can be used as a marker gene for the selection of growth and carcass traits of synthetic chickens in a marker-assisted selection program.