How do I politely refuse to undertake more assignment at work place

If additional tasks will jeopardize any of your current projects, you MUST tell your boss about this, and solicit his/her help in setting priorities. I'm a "boss" but not a mind reader. If I ask someone to do something, and they say OK, I would naturally assume they can and will do it. If they have a problem with it, how am I supposed to know this, unless they tell me? It is a boss's nature to keep piling on the work, until the worker says "enough!" It's wrong to just keep accepting more and more assignments, and let yourself get spread so thin that none of them get done on time, within budget, or with excellence. One day, you'll have program failure, and you'll be in even more trouble for not waving that red flag in time for the boss to do something about it. As far as the "polite" part, you already know how to do that. Do not ask the boss "what part of 'NO' do you not understand?!?" Do not tell him to F*** off. Just level with him (or her) about workload and time constraints.

Yes i also face this type of situation on many occassion ... at my work place... I always try to do my best for assignment to me. however, when i intend to refuse... i normally respond that ... i doubt very much.. due to pre-occupation i will not be able to maintain quality... which i always prefer.. so it would be better not give me the new assignment right now...

Politely tell your boss/supervisor that to take on an additional assignment at this point in time would have a serious affect on the time you have available for the current projects. Also that if you are forced to take on the additional assignment, then the quality of all the work you are undertaking will be reduced.

you must tell your boss about your current work status and tell him it will make early time lines delayed.

I read in an article once that you should ask your boss to help you prioritize all of the tasks that have been assigned to you. This helps you to subtly inform them of everything that's on your plate and perhaps get them to push back deadlines or target dates on that which is not so timely. After all, it's probably not too bad to have a lengthy todo list as long as you don't have impossible deadlines looming. Best of luck to you!