The severity of autism spectrum disorder was strongly related to the education and service outcome. Without a clear profile of autistic population and its change, efforts to understand its nature and improve the quality of service or education will be impossible. The present study aims to describe the over time reported rate of autism severity based on a nationwide population-based 10 years register data in Taiwan. We obtained the national register data--"the Disabled Population by Aged and Grade" and "the Taiwan General Population by Age" to examine the overtime trend change of autism severity from 2000 to 2009. Results showed that the reported prevalence of autistic cases significantly increased from 0.93 to 3.96 per 10,000 population in 2000-2009. The group of mild disability grade was significant increasing from 33.4% to 56.2% in percentage of four disability levels in the year of 2000 and 2009, the other groups were decreasing percentage significantly at the same period (p less than 0.001 in linear trend tests). However, the gender difference in the number of autism in all disability severity needed to be measured in the future study. The study highlights that it is needed to measure the effect of autism severity on the healthcare and education needs in the near future, to improve the quality service for people with autism spectrum disorder. (Contains 1 table and 4 figures.)