The use of Ca(OH)2 pre-treatment increased the biomethane production from all the particle sizes tested in this study. Over the initial 5 days of agenerase the average rate of biomethane production resulting from 3, 2 and 1.25 mm straw was 40.45 NmL-CH4 g-VS−1 d−1. This rate was more than 4-fold the average rate of methane production over the same periods from the milled-only 10 mm straw. Although gas potential yields from the alkaline pre-treated 10 mm wheat straw were significantly lower than all the other particle sizes in the initial 5 days of digestion time, the larger particle size produced a similar amount of gas to the 1.25 mm cut at the 15-day time-point and was only significantly lower than the 2 mm milled particle sizes by day 30. The final biomethane potential yields achieved after Ca(OH)2 pre-treatment of wheat straw with 30 days of anaerobic digestion were 301 ± 4, 313 ± 2, 320 ± 10 and 305 ± 17 NmL-CH4 g-VS−1 from 10, 3, 2 and 1.25 mm milled straw sizes, respectively.