In this study, corrosion behaviour of magnesium AZ31 alloy sheets produced by different casting methods was investigated. Magnesium AZ31 sheets of 2 mm in thickness produced by conventional direct chill casting + rolling and AZ31 sheets of 2 mm in thickness and 1500 mm in width produced by twin roll strip casting + rolling were used as samples. For the determination of processing method dependent corrosion behaviour, potentiodynamic electrochemical corrosion and also immersion tests were performed in 0.5M Na2SO4 and 0.01M NaCl solutions. Corrosion damage occurred during the tests was extensively investigated by scanning electron microscopy. Results of the work revealed the clear role of processing method dependent size and distribution of intermetallics on the corrosion behaviour of AZ31 sheets.