Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy have been employed for crystallographic and microstructural characterization of nano-precipitates distributed within the α-Zr matrix of the Zircaloy-4 (Zr-4) alloy. These nano-precipitates, typically with an average length between 200 and 500 nm, have been identified as the C16-type Zr2Fe phase. A massive presence of randomly distributed planar defects, including stacking faults and sub-unit cell twins, were detected. Atomic-scale configurations of the intrinsic planar defects in the Zr2Fe nanoparticle phase were investigated, for the first time, through high-resolution TEM analysis. Results show that the stacking faults are extended on (110) planes with a displacement vector of 1/4[11¯1]11¯2[33¯2]. HRTEM observations and electron diffraction patterns acquired by specimen tilting reveal that these twin defects belong to 60° rotation twin with the rotation axis along the [110] direction. Geometric analysis based on the the TEM results reveals that the twinning plane and direction are 1/4[11¯1]11¯2[33¯2] and 1/4[11¯1]11¯2[33¯2].Graphic abstract: