Hi
I'm from Korea and a Post-doc of basic science working in the United States.
I have a question regarding the period and eligibility of tax treaty exemption.
Here is my visa history.
I was a student with F1 visa and stayed in the United States for 6 months in 1998.
After that, I have visited USA for a short time like 7-10 days to attend a conference as a B1 visa holder almost every year.
In February 2007, I came to the United States as a J2 visa holder and stayed here for 4 months. In June 2007, I left the United States and changed my visa to J1-research scholar in Korea and re-entered the United States as J1 visa holder.
Currently, my visa is J1.
My question is that am I eligible for two-year income tax treaty exemption?
I wonder that the period 'two-year' means 24 months or just calendar year.
Please give me an answer.

Based on what you have posted, you ARE eligible for the two-year tax exemption. The details for Korea can be found in IRS Pub 901 (Tax Treaties), page 16.

For the tax-exemption, two years means 24 months. You can stay a bit longer to finish up the details of your stay, but do not overdo it, because if the IRS determines you violated the 24-month period, they will retroactively impose the taxes that would have been during that two-year period.

Thank you for your kind answer.
I have one more question.
As I already described in my question, I stayed in the United States as a F1 visa holder for 6 months in 1998. Do the tax treaty also include this 6 months stay in the 24 months period of tax treaty exemption? Or it just starts counting from the arrival date of J1 visa?

No, the F-1 time does NOT count towards this two-year period, especially when you consider that it was almost ten years ago.

The two-year/24-month clock started when you arrived in the U.S. in 2007.

Thank you so much for your answer.
Now, I clearly understand that I'm eligible for tax treaty exemption.
Thanks.

Glad to help!