Purpose: MSR-B is a high-strength magnesium alloy characterized by good mechanical properties both at an ambient and elevated temperature (up to 200°C). It contains silver and rare earth elements. The aim of this paper is to present the results of research on the microstructure and mechanical properties of the MSR-B magnesium alloy in an as-cast condition. Design/methodology/approach: For the microstructure observation, a Reichert metallographic microscope MeF2 and a HITACHI S-3400N scanning electron microscope with a Thermo Noran EDS equipped with SYSTEM SIX were used. A qualitative phase analysis was performed with a JEOL JDX-7S diffractometer. Quantitative examination was conducted using the “MET-ILO” automatic image analysis programme. Findings: Based on the investigation carried out it was found that the MSR-B alloy in an as-cast condition characterized with a solid solution structure α with island of divorced eutectic (α + (Mg,Ag)12Nd and probably (Mg,Ag)41Nd5 phase). The mean area of the solid solution α grain equals Ā=543 μm2, and the mean surface fraction of eutectic regions is AA= 5.81%. The yield strength is near 90 MPa in 20°C and near 70 MPa in 200°C. Tensile strength is near 180 MPa in both temperatures. The material hardness is 47 HV. Research limitations/implications: Future researches should contain investigations of the influence of alloy additives on microstructure and mechanical properties of MSR_B alloy in as-cast condition and after heat treatment. Practical implications: MSR-B magnesium alloy is used in the aircraft industry, for wheels, engine casings, gear box casings and rotor heads in helicopters. Results of investigation may be useful for preparing casting technology of the Mg-Ag-Nd alloys. Originality/value: The results of the researches make up a basis for the next investigations of magnesium alloys with addition of Ag and Nd designed to exploitation at temperature to 300°C.