Design challenges arise in applications like humanoids where high torque is required to move and self-balance the bot. Present research focuses on development of a cycloidal gearbox for humanoid shoulder joints. Light weight gear box assembly was designed with mass of 360g. Structural design and analysis of cycloidal gearbox is carried out. A 3D printed prototype is developed to validate motion analysis. It was actuated by NEMA 17 step motor (0.45 N-m) which is controlled by A4988 motor driver module on the Arduino platform. The speed reduction ratio is achieved 20:1 in a very small space of 60 x 60 x 20mm. Motor torque 0.45Nm is magnified up to 8.82Nm. Result shows proposed cycloidal gearbox satisfies requirement of humanoid shoulder joints such as high torque, light weight and compact assembly. Additional features noticed are back drivable, vibration and noise free transmission which are highly desirable for humanoid shoulder joints.