I am doing real time RT-PCR of human cell line. May I know if the gene expression in different biological replicates will be different or almost the same?
Do you extract RNA, prepare cDNA, and set up qPCR reactions for all of your biological replicates at the same time? If so, I would not expect much of a difference. Having said this, if your biological replicates are from various cell passage numbers, grown under different growth conditions, and harvested at various cell densities, you may see differences in the expression levels for your gene of interest.
You talk about "cell line" and "biological replicate" in the same sentence... do you mean you prepare cell-lines from different individuals? These would be biological replicates. Or do you prepare cultures from the same cell-line stock? These would not be biological replicates.
For real biological replicates there is no general answer. There are billionos of factors that can influence the gene expression and you don't know them and you don't control them. So considerably different expressions are possible.
For technical replicates the variability depends on your experimental design/measurement protocol (all what Mohsan already mentioned). If you have synchronized cultures then the relative time of the measurement can have a huge impact. Also the time of the day can have an impact (-> circardian and other biological rythms).
Regarding differences among technical and/or biological replicates, I found very useful the following publication.
http://www.ncbi.nlm.nih.gov/pubmed/19643838
It explain how to contain the source of confounding variance choosing the best balance between biological and technical replicates in your experiment.
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