I used three light planes as light source - key light, fill light and rim light…and rendered in cycles and it came out as it should be - realistic and fantastic. But now I experimented with darker scene…so I deleted all lights, made an emitting object near my model which will lit the base plane and the object and a very low power key light. My aim was to create scene like the object in only glowing and lighted thing in the room. I rendered with same camera and rendering settings in cycles but the result is gross 
Like some amateur took photo in dark…it’s grainy like hell and so bad.
help please.
So, “fix it in post.” Get a good, clean exposure of the various pieces such that you can after-the-fact and with some tool rebalance them separately to achieve the effect you want. Never consider that the renderer’s output is where the process ends. “First there’s the camera and film, then there’s the darkroom.”
ah i agree with you ^^ post processing is equally important. But for the start, the grains has gone as i rendered in 2000.
thank you for the advises ^^