The thermoelectric properties, electronic band structure, density of states, chemical bonding, optical properties, electrons and holes effective mass of 1,3,6-trimetylo-alloxazine are calculated by using the full potential linear augmented plane wave method, based on the density function theory. Result shows that 1,3,6-trimetylo-alloxazine is an indirect band gap semiconductor. The band gap for LDA, GGA, EVGGA and mBJ are 1.919, 1.984, 2.106, 2.335 eV respectively, the valence bands are mostly attributed to N-s/p, H-s, O-p and C-s electronic states, and the conduction bands are attributed to N-s/p and O-p electronic states. The electrical transport properties and types of carriers of 1,3,6-trimetylo-alloxazine are attributed to N-p and O-p electronic states near the Fermi level. From the electronic charge density nature, the covalency can be clearly seen in the bonds C–N and C–H. From the optical behavior we found that the reflectivity of the investigated compound is smaller. Thermoelectric properties of semi conducting 1,3,6-trimetylo-alloxazine were also reported. 1,3,6-trimetylo-alloxazine compound is approximately large band gap semiconductor with a large p-type Seebeck coefficient. The thermoelectric figure of merit is small because of high electrical resistivity and thermal conductivity.