AZ91 alloy in its molten state oxidizes rapidly in atmospheric conditions and begin to bum from the melt surface. In order to prevent the molten AZ91 alloy from burning, it is usually protected by a cover gas such as sulfur hexafluoride $(SF_6)$. However, the use of $SF_6$ gas poses a serious threat to the environment as it is widely acknowledged to contribute to the global warming effect. Thus, several types of studies aimed at controlling the oxidation behavior of molten magnesium alloys have been carried out. This paper describes the characteristics of the oxide layers formed on molten AZ91 alloys containing Ca and Be. The surface analysis technique of GDS (Glow Discharge Spectroscopy) was used to investigate the oxide layers of AZ91 alloys containing Ca and Be. The GDS results showed that the oxide layer consists of CaO, MgO, BeO and $Al_2O_3$.