Strategic human resource management (SHRM) is an important influence on organizational performance. Previous research has established the positive impact of a specific set SHRM practices – known as high performance work systems (HPWS) - on firm performance. This study furthers our understanding of the performance impact of HPWS by: (a) proposing organisational co-ordination as a unique process-based mediator between HPWS and firm performance and (b) examining the impact of national cultural differences on the relationship between HPWS and organisational performance. Data were collected from 98 Chinese and 93 Irish accounting firms. The findings support the mediating role of organizational coordination in the relationship between HPWS and firm performance. Additionally, we found the effects of organizational coordination were stronger in the high power distance and collectivistic culture of China than in the low power distance and individualistic culture of Ireland. Implications are discussed