Thailand's global ranking on administration by law has slipped nine places to 56 out 102 countries this year from 47th out of 99 countries in 2014, according to the World Justice Project (WJP)'s 2015 Rule of Law Index.
In the East Asia-Pacific region, Thailand is ranked near the bottom at 11 out of 15 countries.
Singapore's rule-of-law performance put it at the top among ASEAN countries, followed by Malaysia. Cambodia was ranked the lowest in ASEAN both this year and last year.
Globally, Denmark leads the list of 102 countries in terms of government by law, while Venezuela came in the lowest.
Factors used to measure the ranking were: constraints on government powers, absence of corruption, open government, fundamental rights, order and security, regulatory enforcement, civil justice and criminal justice.
The index is based on a survey of over 100,000 households and 2,400 experts to figure out how law is applied and enforced in each country, with each nation scored on 47 indicators across eight categories.
Thailand's ranking dropped several places after last year's military coup. According to the WJP index, protection of Thai people's fundamental rights has been affected significantly, making it one of the most worrying issues when it comes to measuring the rule of law.
In terms of government powers, the Kingdom came in at 76th out of 102 countries, though it did better when it came to corruption, coming in at 29th globally - eighth out of 15 countries in region and seventh out of 31 higher-middle income countries.
The WJP index aims at creating an effective rule of law to reduce corruption, combat poverty and disease, and protect people from injustices, as well as ensuring there are more accountable governments and people's fundamental rights are respected.
The agency also encourages citizens and leaders from around the world to play a role and boost public awareness about the importance of the rule of law, while also promoting policy reforms and more on-the-ground development programmes.