In this work porous MgO fibers were investigated as an immobilizing agent in the solid molten salt electrolytes for thermal batteries. Their distribution, uniformity and electrochemical performance were characterized. Compared with ball-milling method, the use of liquid nitrogen as mixing medium resulted in an homogeneous distribution of molten salt particles on the surface of porous MgO fibers. The ionic conductivity of the separator using porous MgO fibers as an immobilizing agent is slightly higher than that of the separator when MgO powders were used as the immobilizing agent. Moreover, the discharge specific capacity of the battery using the porous MgO fibers, 564.39 mAh . g(-1), is significantly greater than that of the battery using MgO powders, 355.67 mAh . g(-1). More importantly, the electrolyte leakage content of the separator based on porous MgO fibers is also the lowest. Porous MgO fibers offer promising application in thermal batteries. (C) 2016 The Electrochemical Society. All rights reserved.