The present invention discloses a method and apparatus for synchronizing weakly ordered write combining operations. A memory controller has a buffer to service memory accesses. A store fence instruction is dispatched to the memory controller. If the buffer contains at least a data written by at least one of the weakly ordered write combining operations prior to the store fence instruction, then the store fence instruction is blocked until a block in the buffer containing the data is globally observed. If the buffer does not contain any data written by at least one of the write combining operations prior to the store fence instruction, then the store fence instruction is accepted by the memory controller.