A nuclear winter is caused by large amounts of dust/particles being thrown into the upper atmosphere and reducing the amount of energy reaching the earths surface. A similar (though less intense) effect has occurred in the past when volcanos ejected lots of dust and particles into the atmosphere. I've heard that after Krakatoa Europe experienced one of it's coldest winters.

Using nuclear bombs to do this is probably on the excessive side of things. This falls into one of the "engineered" theoretical solutions to global warming. Using planes or rockets to disperse highly reflective particles into the upper atmosphere would probably be a more efficient and less damaging method. There are many other ways to artificially increase the reflectivity/albedo of the earths surface that would achieve a similar result. It would most likely need to be continued for as long as there were excess greenhouse gases in the atmosphere.

I think this idea will not work properly as nuclear explosion cause nuclear fall out. After a nuclear explosion tonns of dust and radioactive material mix with atmosphere and fall slowley for long time which cause blood cancer, hemophelia etc.

To create a nuclear winter effect we'll have to explode a lot of bombs in various heights and places to cover earth's atmosphere properly but in this case it may harm our ozone layer also. Another reason is that if we cover whole atmosphere with nuclear shelter then many plants and forests can disappear from earth due to not getting proper light and improper photosynthesis.

In all case it will be a bad idea for solving global warming problem.

It has also been argued, looking at the past geological record, that ice ages follow periods of heightened temperatures. So we might be looking at a cold period a few hundreds years from now (thumbsuck timeline guess on my part) anyway. 

Another thing to consider: A winter induced by blocking the rays of the sun will affect plant life and likely freeze vast areas of the oceans - these are the two main systems that dilute and remove CO2 from the atmosphere. If their capacities are reduced, a winter as you mention would just delay the warming and possibly make it worse once temperatures start to rise again. Notwithstanding that such a winter would devastate our civilization, we would have to produce even less CO2 than the lower standards currently called for. Simply put: int he long term it could make things much worse.

A winter of this sort is not a normal winter as we know it. It will cause wide-scale ecological changes that could arguably change the world even more than global warming would. At the very least it would just mean exchanging one cataclysm for another. 

As long as it does not result in the "year without summer" and the ensuing famines of 1816.