The primary mirror assembly is the key opto-mechanical subsystem of the airborne SOFIA telescope. It consists of a 2.7-m primary mirror and a mirror support structure, the so-called primary mirror cell. The mirror is a monolithic ele-ment of Zerodur with a milled honeycomb structure on the backside. Despite of its size it has a mass of approx. 885 kg only. The mirror cell is a lightweight structure made from CFRP panels and profiles, bonded and riveted together with metallic inserts and joints. It provides an isostatic but stiff mounting of the mirror. The first natural frequencies are pre-dicted to be above 70 Hz for the whole 2000-kg assembly. The paper presents the actual structural properties of the primary mirror assembly determined in a modal survey test as well as the optical performance of the mirror mounted in the cell measured in horizontal and vertical orientation of the optical axis.