Arduino Uno Measurement of Small Capacitance

Hi,

if I want to measure a small capacitance (pF), how do I do it?
I have followed http://arduino.cc/en/Tutorial/CapacitanceMeter before and find that it does not work well in small capacitance.
How do I get a very small resolution like 0.001pF?

Measuring capacitance in that range is extremely difficult. PCB traces, test leads, etc etc have capacitances that are three orders of magnitude higher than what you are trying to measure. These capacitances also vary significantly with temperature, humidity, etc etc making nulling them out very difficult. You are also fighting against stray inductance in traces, wires, component leads, etc etc.

If you come up with a cheap, accurate way to do it with an Arduino, start a test equipment company.