Background: Although there has been considerable research into the reliability of marking for the Key Stage 3 (KS3) National Curriculum tests (NCTs) and public examinations such as the General Certificate of Secondary Education examinations (GCSEs) in England, little is understood about the level of reliability of marking of the Key Stage 2 (KS2) NCTs taken by pupils aged 11, particularly the KS2 English reading and writing tests. Purpose: This study was intended to investigate the reliability of marking of the National Curriculum KS2 English writing tests using a range of statistical methods. Methods: The KS2 English writing test assesses a range of pupils' writing skills. Pupils' scripts are marked by human markers. Markers are trained and undergo a marking standardisation process before live marking to ensure marking consistency. The quality of marking is further monitored through the embedding of benchmark scripts in live marking. Data collected from marker standardisation and benchmarking for the KS2 English writing tests administered in 2010 and 2011 were analysed in this study. Results: Considerable variability was found in the marking of the KS2 English writing tests administered in 2010 and 2011 at both question paper level and individual task/marking strand level, reflecting the nature of the tests. The quality of pupils' work to an extent affected the consistency in marking between markers. There was no substantial variation in marking consistency over time for the markers. Conclusions: The reliability of marking of the KS2 English writing tests investigated is comparable with that of assessments of similar nature and is at the expected level. The present study provided a better understanding of the level of marking reliability of the KS2 English writing tests.