Millimeter-wave (mmWave) communication is the rising technology for simultaneous wireless information and power transfer (SWIPT) unmanned aerial vehicle (UAV)-based relay networks for the abundant bandwidth and short wavelength. However, the secrecy performance of mmWave SWIPT UAV-based relay systems has not been investigated so far. In this paper, we consider secure transmissions of mmWave SWIPT UAV-based relay systems in the presence of multiple independent homogeneous Poisson point process (HPPP) eavesdroppers. Different from most existing works considering a free-space path loss model, the air-to-ground channels are modeled as Nakagami-m small-scale fading, and the effects of blockage to mmWave links on the ground and the 3D antenna gain model are considered. The closed-form expressions of the average achievable secrecy rate and energy coverage probability for amplify-and-forward (AF) and decode-and-forward (DF) relay protocols under power splitting (PS) SWIPT policy are derived to reveal the impact of various parameters on system performance. The simulation results show that there is the optimal UAV position to maximize the secrecy rate and the optimal position is closer to the node whose transmit power is relatively small. Furthermore, different carrier frequencies are suitable for different eavesdropping node density. INDEX TERMS Millimeter-wave (mmWave), UAV relay, SWIPT, physical-layer security. I. INTRODUCTION Low-altitude unmanned aerial vehicles (UAVs) are becoming more and more attractive for their high mobility, low-cost and flexible deployment. UAV-enabled Internet-of-things (IoT) network is a promising application with the widespread use of smart devices [1]. In some scenarios, UAVs need to support high data rate requirements, e.g., stream large real time video monitoring traffic data and high quality pictures to ground nodes. To support high data rate requirements, mmWave communication becomes a key enabler in UAV-enabled IoT network for its large available bandwidth [2], [3]. Since mmWave signals are particularly sensitive to blocking, communications on the ground will be interrupted when there are large obstacles such as hills or large buildings. Compared with the terrestrial systems, UAVs can easily move from one place to another to avoid blockages, which can maintain The associate editor coordinating the review of this manuscript and approving it for publication was Kuo-Hui Yeh.