Loading path changes affect forming processes markedly. This matter is getting more important especially in metal forming process in which engineers face unexpected results after drawing processes. As a result, understanding of formability changes through strain-path changes is highly demanding. This paper investigates the effect of strain path change on the forming limit diagram (FLD) of 5083 aluminium alloy sheet experimentally and theoretically. The experimental method is a new approach for determining the FLD under nonproportional loadings. The aim is to understand the formability of this alloy under various strain paths. For this purpose, out-of-plane formability test method with a hemispherical punch was used. Considering trim operation during forming process will simulate the changes in loading. Furthermore, forming limit diagram was determined theoretically by using the well-known Marciniak and Kuczynski (M-K) method. The results showed that uniaxial prestraining increases and shifts the FLD to the left-hand side of the diagram for both parallel and perpendicular to the rolling direction. Biaxial prestraining shifts the FLD to the right-hand side of the diagram for both directions and also decreases the forming limit curve (FLC) for the specimens parallel to the rolling direction. Theoretical results showed good agreements with experimental observations.