Why Serial.println() modify my data?

Porama6400:
I have some raw byte data, I'm too lazy to convert them to Hex so I decided to just use println() directly on it

Turn out it modify that data after println() is called, I've test it several times and result are same, Those data are modified by that println() command

why is that?

Debugging hint: look around the forum.
How many posts report that Serial.println is corrupting data?
Not many, right?

Clue: guess where the problem lies.