In this study, hollow Co, Fe@air@Co and Fe@Co nanoparticles (NPs) have been synthesized respectively by electroless plating Co shell on Fe core and controlling reaction time based on galvanic cell reaction between Co shell and Fe core in hydrochloric acid at room temperature. The electromagnetic (EM) wave absorption properties of these three NPs are also been investigated. The results indicate that the relationship between Fe core and Co shell is critical to the EM wave absorption properties of hollow Co, Fe@air@Co and Fe@Co nanoparticles when blended with 70 wt% in paraffin-based samples. Fe@air@Co nanoparticles shows the best EM wave absorption properties with minimum reflection loss of -42.75 dB and effective bandwidth of 4.1 GHz under -10 dB. The present work has a significant potential for the development of EM wave absorbing materials with core-shell structure. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.