The objective of this paper is to determine ammonia (NH3) volatilization factors (as a ratio of volatilized NH3-N to applied ammonium nitrogen [NH4-N]) following the application of dairy cattle slurry to grassland surface based on the results of a number of pot and field experiments. Pot experiments examined the effects of both environmental factors (soil water condition and air temperature) and properties of the slurry (dry-matter content, NH4-N content and pH) on NH3 volatilization loss from slurry applied to the soil surface in a pot where grass was grown. A grassland field trial was also carried out to confirm the results from the pot experiments. Our results demonstrated that a slight application rate (<60 Mg ha−1), dry soil water conditions, low air temperature, low NH4-N content in the slurry, low dry-matter content and low pH of the slurry, and acidification by adding superphosphate to the slurry, could abate volatilization. Our experiments also showed that the volatilized NH3-N was basically determined by the application rate of NH4-N from the slurry, even though the slurry was applied at different rates and under conditions that affect NH3 volatilization. From these results we estimated the cumulative volatilization loss of NH3 from the surface-applied slurry from the time the slurry was applied until volatilization was complete and then calculated the volatilization factor. It could be deduced from this calculation that the recommended values of the volatilization factor (g NH3-N [g NH4-N]−1) from the surface-applied slurry were as follows: 0.32 when the application rate of the slurry was less than 6 kg m−2 (60 Mg ha−1); 0.42 when the rate was more than 6 kg m−2; 0.37 when the rate was unknown.