High-density polyethylene (HDPE)/mica blends with varying particle sizes and diameter-to-thickness ratios were prepared using the melt blending method. The microstructure, mechanical, and thermal properties of the blends were characterized and analyzed using a scanning electron microscopy (SEM), a universal testing machine, and a differential scanning calorimetry (DSC). The results showed that both 400-mesh and 2 500-mesh mica particles could be uniformly dispersed within the HDPE matrix. The tensile strength of the blends reached a maximum value of 65.5 MPa when the 400-mesh mica additive amount was 10 phr, and the flexural modulus of the blends was as high as 807.0 MPa when the 400-mesh mica additive amount was 20 phr. The impact strength decreased with the increase of mica additive amount. The melting temperature, crystallization initiation temperature, and heat deflection temperature slightly increased with the increase of mica content. The melting flow rate of the 400-mesh mica blend decreased to 2.1 g/10 min when the 400-mesh mica additive amount was 15 phr.