Timeline of applying for US citizenship.
I obtain my greencard on Oct, 2000. I obtained it via H1B visa. I have
never left the US for more than six months consecutively and my total
time of staying outside of the US since Oct, 2000 is less than nine
months. I dont have any criminal records. I know for sure I am not
qualified for applying for citizenship after on permanent residency
for three years.

My question is how soon can I file my US citizenship application ? Do
I have to wait until Oct, 2005 or I can file my application now ?

Hi cweng,

Assuming you are not married to a U.S. citizen, and you meet the
"continuous residence" requirement, and the date on your Permanent
Resident Card is Oct 1, 2000, you may send in your application July 2,
2005 (I would do it July 4 to be on the safe side).

A Guide to Naturalization
When does my time as a Permanent Resident begin?
"Your time as a Permanent Resident begins on the date you were granted
permanent resident status. This date is on your Permanent Resident
Card (formerly known as Alien Registration Card). The sample cards on
this page show where you can find important information like the date
your Permanent Residence began."
"If you are applying based on 5 years as a Permanent Resident or 3
years as a Permanent Resident married to a U.S. citizen, you may file
for naturalization up to 90 days before you meet the ?continuous
residence? requirement. For example, if you are applying based on 3
years of ?continuous residence? as a Permanent Resident married to a
U.S. citizen, you can apply any time after you have been a Permanent
resident in continuous residence for 3 years minus 90 days. You may
send your application before you have met the requirement for
?continuous residence? only. Therefore, you must still have been
married to and living with your U.S. citizen spouse for 3 years before
you may file your application. You must also meet all the other
eligibility requirements when you file your application with USCIS."
Continuous Residence 
?Continuous residence means that you have not left the United States
for a long period of time. If you leave the United States for too
long, you may interrupt your ?continuous residence.? "If you leave the
United States for more than 6 months, but less than 1 year, you have
broken or disrupted your ?continuous residence? unless you can prove
otherwise. Read the ?Document Checklist? in the back pocket of the
Guide to find out what information you must give to us to prove you
did not break your ?continuous residence.?
http://uscis.gov/graphics/services/natz/English.pdf

Additional Links of Interest:

Naturalization Eligibility Worksheet
http://uscis.gov/graphics/services/natz/wsinstruct.htm

N-400, Application for Naturalization
http://uscis.gov/graphics/formsfee/forms/n-400.htm

I hope this helps. If you have any questions, please post a
clarification request *before* closing/rating my answer and I'll be
happy to reply.

Thank you,
hummer

I searched the Guide to find the answer.