Hi Niwantha, Both are adverbs that refer to a time before the present moment. In my opinion, the choice of either word depends on the way your sentence is constructed. Example: 1. The flight departed ten minutes ago (not back). You will notice that when you use "ago", you need to indicate how far in the past something occurred before the word "ago". 2. A long time ago (not back), a prosperous kingdom existed. Again, we indicated how far in the past the kingdom existed. Example: 3. The economic depression hit sometime back (in 2007). 4. Sometime back (in the eighties), rock and roll was very popular. You will notice that "back" is usually followed by "in... (year, period, decade etc)" On the contrary, it sounds very unnatural to write: The meeting ended twenty minutes back. /The meeting ended sometime back. OR Ago in January last year, there was a snowstorm. / Sometime ago in 2008, the king abdicated from his throne. Hope this helps. Cheers, Lance