Light weight, high speed, compact power transmission shafts transmit torque through diaphragm type flexible discs whose planes are normal to the torque axis. The angular and axial misalignments are obtained by material flexure of these thin diaphragms within elastic limits. These shafts have multiple diaphragms used in series so that each diaphragm shares a portion of the total misalignment. Such shafts are used in high speed aircraft coupling engine mounted accessory gear box and aircraft mounted accessory gearbox. This paper deals with the design and development of light weight flexible power transmission shaft for high speed fighter aircraft application keeping critical speeds and weight as the design criteria.Since the number of design constraints is more, finite element analysis has been used as an effective tool for trying out various design iterations so that all the design constraints are met. Initially a configuration of the PTO shaft has been designed based on the torque capacity and envelope specified and a finite element model has been developed. The PTO shaft being made of thin walled cross sections, it is modelled using quadratic shell elements. The inertias due to AMAGB and Engine are lumped as inertia elements at appropriate locations. Based on the torque carrying capacity and three critical speed requirements, number of analysis/design iterations have been made before arriving at the final design configuration accommodating all the design constraints. The analysis results of the final design have been validated using simplified analytical math models. The effect of centrifugal stiffening, and manufacturing tolerance on the critical speeds and the misalignment stress have also been studied [1,2]. Based on the design, the prototype of the power transmission shaft has been made and has successfully undergone development tests. The critical speed of the shaft estimated using modal testing has been compared with the ones obtained using finite element analysis .