The paper presents a single constraint equ ation of the direct kinematic solution of 6-dof (Stewart-Gough) platforms. Many resear ch works have presented a single polynomial of the direct kinematics for several 6-dof platforms. However, the formulation of the polynomial has potential problems such as complicated formulation procedures and dis crimination of the actual solution from all roots. This results in heavy computatio nal burden and time-consuing task. Thus, to ove rcome these problems, we use a new formu-la tion approach, called the Tetrahedron Appr oach, to easily derive a single constraint equation, not a polynomial one, of the dir ect kinematics and use two well-known numer ical iterative methods to find the solution of the single constraint equation. Their pe rformance and characteristics are investiga ted through a series of simulation.