The vertical wind tunnel, which has been designed for sky-diving operations, consists of an octagonal to square diffuser. The diffuser configuration indicates that the wall angles in the diffuser are not symmetrical: the smaller and larger angles being 3.43 and 10.66 degrees, respectively. Estimation of pressure drop and study of flow separation in the diffuser is of great importance for the design of the vertical wind tunnel. In this research work, a 7.8 % model of the original diffuser has been constructed, and airflow, in both blower and suction modes, has been studied in the diffuser using electronic pressure gauges. Pressure drop in the test diffuser has been measured and compared with pressure drop in a circular to square diffuser, which is installed in the vicinity of the test diffuser. To estimate the pressure drop in the test diffuser, the larger diffuser angle from the above, namely 10.66 degrees should be used in the semi-empirical equations, and use of the equivalent diffuser angle is not recommended. In addition, study of the pressure recovery coefficient showed no significant flow separation in the tested diffuser.