user14503: Assume I traded 100 times between BTC and LTC and I earned $100 IF I sell all coins at market price.
However, I don't want to sell my coins yet. How I can report to the US tax?   

user51948: 
The IRS treats digital currencies as capital assets and are therefore subject to capital gains taxes. Any disposition of these digital currencies, including trading and spending, is a tax event and capital gains must be calculated in USD. 

References:
https://en.bitcoin.it/wiki/Tax_compliance

14503: <p>Assume I traded 100 times between BTC and LTC and I earned $100 IF I sell all coins at market price.</p>

<p>However, I don't want to sell my coins yet. How I can report to the US tax?   </p>
 51948: <blockquote>
  <p>The IRS treats digital currencies as capital assets and are therefore subject to capital gains taxes. Any disposition of these digital currencies, including trading and spending, is a tax event and capital gains must be calculated in USD. </p>
</blockquote>

<p>References:
<a href="https://en.bitcoin.it/wiki/Tax_compliance" rel="nofollow noreferrer">https://en.bitcoin.it/wiki/Tax_compliance</a></p>
