Section 118 of the Indian Evidence Act states that any person can be a witness. Witnesses must be capable of understanding the question and give a rational answer to that question. Mental condition of the witness is the most important thing. He may be a handicapped such as deaf or dumb. If he can assist the court and testify himself then he is a competent witness.
Convicted person can be a competent witness
The court first tests the veracity of witnesses. If the court, after putting some questions to the witness, finds that he is reliable and will state the truth then the court will allow him to depose himself as a witness. Conviction does not harm the truthfulness or veracity of a witness.
The court takes utmost care while taking the evidence of a convicted person. If the evidence of a convicted person is corroborating with other evidence or his evidence seems trustworthy the court will not deny his statement. In Veerappan vs State  the Supreme Court has held that “It is the duty of the court to carefully examine and scrutinise the evidence.”
Inherent defects in statement of convicted person
Criminals are very shrewd people. He can depose the false statement very cleverly and mislead the court. Some convicted people have no fear of imprisonment or sentence hence, they depose false evidence. These are the inherent defects in admitting evidence of a convicted person.
Reliability of convicted witness
If a witness is reliable and states the truth, the court does not reject his evidence. The witness states what he saw, heard or perceived by senses any information related to the offence. If that information is relevant and admissible under law then witnesses can depose that evidence. It does not matter that the witness is a reputed person of the society or a convicted criminal.
Status of witness is immaterial. His reliability and ability to state the truth is more significant. Hence, a convicted person can be a witness in a criminal trial. The court will admit his evidence if it seems true and relevant. The Indian Evidence Act does not bar a convicted person to appear as a witness of fact.