The UN Human Rights Special Envoy for Cambodia is making his third visit to the country.
Surya Subedi plans to look at the country's judicial system, which has long faced allegations of corruption and political interference.
A pressure group says that defendants are often denied their basic rights.
Last year, Professor Subedi said that bringing criminal charges against critics of the government was "disproportionate and unjustifiable".
The visit comes a week after the Supreme Court found a prominent opposition figure guilty of defaming Prime Minister Hun Sen.
A succession of land disputes have meanwhile sparked protests by poor villagers, who argue that judges tend to favour well-connected individuals.
The Centre for Justice and Reconciliation, a local organisation funded by the UN, has also sought to drawn attention to the plight of defendants.
It says that basic judicial rights, including the right to be tried in person, are frequently not met.
Of trials held in absentia, it says, nine out of ten trials in absentia last for less than 30 minutes, and many call no witnesses.
It is potentially tricky territory for Professor Subedi, who - unlike his predecessors - has maintained a good relationship with government officials.
But he seems to be able to deliver criticism in a way that does not put noses out of joint.