Abstract: This in vivo experiment was carried out at Sakhon Nakhon Rajabhat University, Sakhon Nakhon Province, Thailand during March-October 2013. The study aims to search for some possibilities in inducing a large number of tetraploid sets of chromosomes in tangerine citrus seedlings with the use of colchicine chemical. A Randomized Complete Block Design (RCBD) with four replications was used. Seeds of tangerine citrus were treated with colchicine solutions. The experiment consisted of seven treatments, i.e., T1 with 0.0% colchicine (control), T2 with 0.2% colchicine solution and submerged for 12 h, T3 with 0.2% colchicine solution and submerged for 24 h, T4 with 0.4% colchicine solution and submerged for 12 h, T5 with 0.4% colchicine solution and submerged for 24 h, T6 with 0.8% colchicine solution and submerged for 12 h, T7 with 0.8% colchicine solution and submerged for 24 h. The experiment was conducted for 91 days. The results showed that colchicine compound severely and significantly affected germination of tangerine citrus seeds. Colchicine of 0.2% in the solution with seeds submerged for 24 h gave the highest percentages of tetraploid chromosomes in seedlings of tangerine citrus (63.64%). Colchicine significantly affected seed germination, plant height, stomata density and leaf index of the tangerine citrus seeds and seedlings. Colchicine had no significant effect on poly-embryos, mono-embryos, leaf number, leaf area, leaf weight, leaf length and stomata length of the tangerine citrus seedlings.