High coercivity and magnetic viscosity in nanocrystalline YCo5 alloy were observed. The alloy was obtained by arc-melting the raw materials, mechanical milling for 4 h with subsequent heat treatment for short times at 1103 K and a final quenching in water. The x-ray diffraction pattern showed a single hexagonal 1:5 phase. The average crystallite size determined by transmission electron microscopy was 13 nm. Magnetic measurements were carried out at 85 K in a pulsed field magnetometer varying the field in the range 50–250 kOe and keeping the pulse duration equal to 0.3 s. A maximum coercivity of 50 kOe was measured when applying a maximum magnetic field of 250 kOe. The viscosity parameter was measured varying the dH/dt rate in the range 511–1113 kOe/ s. The calculated activation volumes from viscosity data were smaller than the crystallite volume from transmission electron microscopy, consistent with their different physical meanings.