how to seal high pressure microfluid with thread?

Dear Tom Song:
Having the threads made with the same tolerances, thread forms, etc. will make all the difference in the world. And it is the worst possible method of sealing. I strongly recommend engineering an o-ring seal, or simply using one of the standard methods that already incorporates an o-ring. SAE is an example of the latter. There is a company that makes a face-seal o-ring, but this is proprietary.
In direct answer to your question, assuming we are speaking of a tapered pipe thread, the sealing occurs on exactly the peak of one full thread, and then across the "valley" between the two "ends" of the fully seated thread. Which is why thread sealing is a really bad idea... too much chance of damaging a knife edge.
David A. Smith

American Standard taper pipe threads (ASA B2.1) use a tapered thread for the male connection and a straight thread for the female connection. As the result, a single thread is (generally) the only one that actually seals. The nature of the process to cut threads is NOT accurate enough and the thread is NOT smooth enough to guarantee that seal requiring the use of tape or paste sealant. For the few threaded connections (shut off valves) that I have in my home, I make certain that I use two wraps of tape on the thread. The tape works better than paste for me.
You may look into *Dryseal* threads. See this page:. You did not define *pressure* with a value so this may or may not apply to your situation. Threaded connections can not be trusted for high pressure use. We used ASTM A106 grade B pipe, welded fittings and SAE 4 bolt flanges for our high pressure applications. For your application pressure, check with the fitting manufacturer for the rated maximum pressure of the fittings. Remember when you *cut* a thread in pipe, you create a high stress area in the thread area in addition to reducing the wall thickness.
Jim Y