I draft entire documents/letters in my office on daily basis but my speaking power is not excellent. how can i improve it?

Faridullah, you are obviously most of the way there, if you have a friend in the office whose spoken English is better than yours read him the letters and documents that you write or make a point of reading them out loud to yourself, practice makes perfect.

Speak the language.....you may be a good writer but you need to speak it on a regular basis.It instils confidence and you get to realize your weak grammatical areas.

Speaking is somewhat an independent skill that must be developed in itself, the best way to do that is through, after taking some courses in communication, much practice of real conversations with native or fluent speakers of English language. 

In order to gain fluency in a language one must practice. We develop our language through speaking with others, sometimes however, it's not possible to find a native to practice with. A perfect solution to this is none other than the infamous Internet. Watch videos, listen to the news, watch TV, fluency is developed over time and with practice! While watching TV or a programme or the news, pause it and repeat it to yourself.

you must talk with native speakers.