But ! after i have connect my resistor, my INPUT was CHANGED ! it down to around 5.6V but not 6V anymore. May i know is there any solution could solve this problem?[/QUOTE]
I can't imagine that the input voltage of the divider can be effected from the input impedance of an ADC stage, there are high enough.
Your problem is that you are using low resistor values and while this is good for the adc it makes thinks at the source side more difficult.
Your source (the 6v) can't drive a resistance that is low (the output impedance is not very low probably) so you have a voltage drop, you can either increase the resistor values (check your ADC specification first for the recommended input impedance) or add a buffer stage between the resistors and source.
Alex