Abstract: The effects of temperature and seed size on germination of tropical pioneer species have received little attention. This study aims to investigate the effects of temperature and seed size on germination of five common tropical pioneer species in Brunei Darussalam; the native Melastoma malabathricum, Melastoma beccarianum, Rhodomyrtus tomentosa, Dillenia suffruticosa and the invasive Acacia mangium. The species were grouped into small-seeded (seed mass ≤1 mg) and large-seeded (seed mass >1 mg) species and we determined the germination responses (number of days for seeds to start germination and reach 50% germination), final germination percentages and Relative Growth Rates (RGR) in different temperature using incubator chambers with a 12 h photoperiod. All species were treated with constant temperatures of 20, 25, 30, 35 and 40°C and alternating temperatures of 30/25, 35/25 and 40/25°C. Results demonstrated the possible influence of temperature on the germination of large-seeded A. mangium (13.86±1.17 mg) as revealed by its fastest time to germination and 50% germination across all temperature treatments, highest mean cumulative germination percentage (88±2.8%) and fastest RGR at constant temperature of 30°C. The abilities of A. mangium to have faster germination and higher RGR than other native plants are the possible traits that made non-native A. mangium invasive in the secondary and degraded forests of Brunei Darussalam. This study also shows potential effects of various environmental factors on seed germination that might contribute towards establishment and growth of pioneers in a habitat.