Technological developments are increasing towards the development of various industries in the world, one of them in the field of material. The use of metal or alloy metal as a material in production began to be abandoned, because metal or alloy metal has a limited lifetime, ease of corrosion and high costs. Natural fiber composites have many advantages such as lightweight, high specific strength, corrosion resistant, and economical. The purpose of this study is to study the tensile, flexural and thermal properties of hybrid untreated-sisal/carbon/ PMMA composites as an alternative material for teeth and internal prosthesis. Fabrication of hybrid randomly metode fiber composites was carried out using hand lay-up technique and in a cold press machines, at room temperature and compressive pressure of 2.1 and 3.5 MPa for tensile and bending test speciment respetively for 60 minutes. Untreated sisal fiber and carbon fiber treated by soaking in nitric acid concentration of 68.3% for 48 hours. The length of raw sisal fiber and carbon fiber is 6 mm. The a fiber/matrix ratio is 20:80% by weight. All hybrid composite specimens are of 2:1, 1:1 and 1:2 ratios of untreated sisal/carbon. Tensile and flexural tests for were refering to the ASTM D638-01 and ASTM D790-02, respetively. In addition, the specimens fracture thermogravimetric (TGA) tested and subjected to scanning electron microscopy (SEM) and fracture of the flexural test results with an optical microscope. The results showed that untreated sisal/carbon/PMMA hybrid composites increased their mechanical strength as the carbon fiber countent increased, the highest tensile and flexural properties were obtained at the sisal/carbon ratio 1:2 with the tensile strength and tensile modulus 57.66 MPa and 3.78 GPa respectively, while the flexural strength and flexural modulus are 112.27 MPa and 5.69 GPa. The results are also explained from the physical test of the fault structure of tensile and flexural tests with TGA, SEM and macro photographs of each showing better thermal stability, structure and bonding fiber matrix.