Missile Onboard Computer is an embedded computer to achieve control and guidance. The Onboard Computer has real-time embedded software that must be designed properly to achieve real time demands for guided missiles. Embedded software architecture commonly has two types: single processing and multiprocessing. Multiprocessing software is faster and more reliable. Multiprocessing software has two common types: asymmetric and symmetric. Symmetric software is the most common configuration for multiprocessing systems requirements. This paper introduces a design of the real time symmetric multiprocessing software for the Onboard Computer of a guided missile. The real time demand for guided missiles can be defined as the execution time for the mission program with its all subroutines must not exceed 10ms and the mission program must be repeatedly executed every 10ms. In this paper, the execution time for the designed symmetric software was 7ms which means that the designed symmetric software is reliable and fulfills the real time demand.