Separation of residual charged particles from neutral particles is typically made by a deflection magnet in a neutral beam injection line for heating fusion plasmas. Ionization of energetic neutrals and neutralization of energetic ions in the deflection magnet region due to collisions with the background gas molecules result in a finite amount of particles that are only partially deflected, which we call a ’’stray particles loss.’’ Quantitative determination of the stray particle loss has been made from calorimetric data of beam power deposited along a beam line employing a 40‐kV 60‐A hydrogen ion source. The stray particle loss is typically 3–5% of the accelerator power and roughly proportional to the background gas pressure. A simple calculation is shown to be in fair agreement with the measurements.