I read that Kindle Fire HD has its own Oxford dictionary, however; I mistakenly deleted that file in the first place when I brought my toy back home. Therefore, I never have a chance to use that function on my device. Recently, I discover a method to use a different dictionary for reading book. I had a English-Vietnamese dictionary to use, so it means that you can use any dictionary you want in your Kindle Fire besides Oxford dictionary.
Step 2: Add a suitable dictionary from Mobile System.
It works like a charm.