Tadalafil is a phosphodiesterase 5 inhibitor used orally as solid dosage form. The suspension of this drug has been used for pulmonary arterial hypertension treatment in pediatrics. The aim of this work was to investigate the influence of non-ionic surfactant (Tween 80) on the physical characteristics, drug particle size, and stress-shear rate rheogram of tadalafil 0.1% suspension. Several formulations prepared by xanthan gum as suspending agent. Glycerin has been used as wetting agent. The several amounts of Tween 80 were added and the characteristics of suspensions were evaluated during 6 months. The results showed the effect of surfactant on sediment volume and resuspendibility. The particle size of tadalafil was affected by surfactant amount. This result showed an optimum effect of Tween 80 on drug particle size. The viscosity behavior evaluation of tadalafil 0.1% suspension showed Tween 80 effect. This study showed that Tween 80 can dramatically affect viscosity of suspensions. The results of this study have demonstrated the effect of Tween 80 on physical properties of tadalafil 0.1% oral suspension. An ideal drug particle size was observed in particular amount of Tween 80 (0.15% w/v).