I want to sell a Canadian stock from my investment account that I bought a month ago. The stock makes about 20% gain and I am afraid it might go down in the further. If I sell it now, will Canada Revenue Agency consider that 20% gain as capital gain and only tax on half of it.
You don't have to wait. If you sell your shares now, your gain can be considered a capital gain for income tax purposes.
Unlike in the United States, Canada does not distinguish between short-term vs. long-term gains where you'd pay different rates on each type of gain. Whether you buy and sell a stock within minutes or buy and sell over years, any gain you make on a stock can generally be considered a capital gain.
I said generally because there is an exception: If you are deemed by CRA to be trading professionally -- that is, if you make a living buying and selling stocks frequently -- then you could be considered doing day trading as a business and have your gains instead taxed as regular income (but you'd also be able to claim additional deductions.) Anyway, as long as your primary source of income isn't from trading, this isn't likely to be a problem.
Here are some good articles on these subjects: