The internet and social media are certainly a great thing for people and society in general, but they are also a uniquely effective breeding ground for potentially libelous statements. Many people have learned (to their dismay) that the internet allows people to speak their mind almost too easily. The internet is chock-full of interesting web sites where someone could intentionally or accidentally leave a potentially defamatory comment or post.
Key issue of online defamation is under Thailand Computer Crime Act (2007) Chapter 1 Computer-Related Offences: Section 14. If any person commits any offence of the following acts shall be subject to imprisonment for not more than five years or a fine of not more than one hundred thousand baht or both:
that involves import to a computer system of forged computer data, either in whole or in part, or false computer data, in a manner that is likely to cause damage to that third party or the public;
that involves import to a computer system of false computer data in a manner that is likely to damage the country's security or cause a public panic;
that involves import to a computer system of any computer data related with an offence against the Kingdom's security under the Criminal Code;
that involves import to a computer system of any computer data of a pornographic nature that is publicly accessible;
that involves the dissemination or forwarding of computer data already known to be computer data under (1) (2) (3) or (4).
Also Thailand Criminal Code can be applied on the false computer data that is the defamation content in SPECIFIC OFFENSES TITLE XI OFFENCE AGAINST LIBERTY AND REPUTATION Chapter 3: Offence of Defamation
Section326. Defamation: Whoever, imputes anything to the other person before a third person in a manner likely to impair the reputation of such other person or to expose such other person to be hated or scorned, is said to commit defamation, and shall be punished with imprisonment not exceeding one year or fined not exceeding twenty thousand Baht, or both.
Section327. Defamation to the Family: Whoever, imputing anything the deceased person before the third person, and that imputation to be likely to impair the reputation of the father, mother, spouse or child of the deceased or to expose that person hated or scammed to be said to commit defamation, and shall be punished as prescribed by Section 326.
Section328. Defamation by Publication: If the offence of defamation be committed by means of publication of a document, drawing, painting, cinematography film, picture or letters made visible by any means, gramophone record or another recording instruments, recording picture or letters, or by broadcasting or spreading picture, or by propagation by any other means, the offender shall be punished with imprisonment not exceeding two years and fined not exceeding two hundred thousand Baht.
Therefore, the police can investigate the online defamation content can prosecute offenders by using these two legislation.