Yes, it is possible to fail a PhD thesis defense. While most PhD students pass their defense, there are still a significant number of students who fail each year.

Here are some of the reasons why a PhD student might fail their thesis defense:

• The student’s thesis is not well-written or does not meet the required academic standards.
• The student does not have a good understanding of their research topic or is unable to answer questions from the committee.
• The student’s research is not original or does not make a significant contribution to the field.
• The student is unable to defend their research effectively during the defense.