You find out about a fraud an employee committed and inform your superior about it with evidence, he didn't take any action. What will you do?

Its not an easy situation.
In case i will ever reach this context, and no action will be taken by my supervisor/direct manager...
I will actually consider to quit, and find another job.
Its against my values, as human, and i would not be able work in a such environment where this kind of behavior is allowed.
It will be clear for me that i will not e able continue my work there, if my supervisor will not take any decision to solve this incident, and will let things like they are....Then again this is strictly my ethics...
And what right means for me !

Policies and procedures have to be followed e.g. wistle blowing processes.

You have fulfilled your duties documenting with evidence at your level.

Related decsion makers areas are not your responsibility. After reporting there are set procedures conducting inquiries audits etc.

So let the action take its due course.

If the convict is in your jurisdiction then as a manager you can take appropriate steps-within your jurisdiction- e.g. changing assignment etc.

We can check with our boss why no actions are taken and we have people above him. Quitting the job is rediculousely foolishenss.

it is my duty to enlighten the incidence to boss, the decision shall lies on him, but i should continue such practices always......

IN THAT CASE, I'LL CALL THE PARTICULAR EMPLOYEE AND ADVISE HIM AND MAKE HIM UNDERSTAND THE IMPORTANCE OF THE WORK AND REPUTATION OF THE COMPANY AND THE IMPORTANCE OF HIS ROLE IN THAT AREA AND WILL MAKE HIM TO REALIZE HIS MISTAKE AND MAKE HIM VOLUNTARILY GO TO THE MANAGEMENT AND APOLOGIZE FOR THE MISTAKE HE HAS COMMITTED.
INSPITE OF ADVICE THE EMPLOYEE IS NOT WILLING TO ACCEPT THE FRAUD HE HAS COMMITTED, I'LL CALL THE ENTIRE TEAM FOR A MEETING ALONG WITH THE PERSON AND TAKE SUGGESTION FROM THE REST OF THE STAFF FOR HIM TO REALIZE THE SAME AND APOLOGIZE.