We have successfully developed, for the first time, a new memory test system for STT-MRAM at wafer-level where an electromagnet is combined with a memory test system and a 300 mm wafer prober. In the developed memory test system, an out-of-plane magnetic field up to ±800 mT can be applied on 10x10 mm2 in the 300 mm wafer with distribution of less than 2.5%. We demonstrated that the electromagnet can apply large enough magnetic field to evaluate magnetic immunity properties for STT-MRAM using 2Mb STT-MRAM; magnetic field dependence of pass-bit rate for ″0″/ ″1″ states, read/write shmoo, and ″0″/ ″1″ retention. All the properties can be explained by general theory for STT-MRAM. The developed memory test system with the electromagnet is a key testing tool for STT-MRAMs, which will contribute to increase efficiency of STT-MRAM testing as well as widening the application area of STT-MRAM sensitive to an external magnetic field.