implied volatility - 引伸波幅

Firstly there is no risk free interest rate in the world because there is default risk even for US treasury bills but we normally assume it is zero.

As a market common practice you would use the interest rate issued by the central government.

When you analyse the call option please bear in mind that you should use the risk free interest rate of that location. Say if you buy HK stock you should use the Exchange Fund Bill rates provided by Hong Kong Monetary Authority.

Also, another thing you need to notice is that when you apply Black-Scholes formula the interest rate is short rate, ie the very short term interest rate. So you should use overnight interest rate. In HK, I think overnight HIBOR or 7-day Exchange Fund Bill rate is good enough.

Actually you do not need to care too much about the interest rate (say 4% or 4.5%) because the interest rate sensitivity to a stock option is small.

Normally interest rate of US Notes is used as risk free interest rate.

US Notes mainly have 3 categories: 2-year Note, 5-year Note and 10-year note.

Their interest rates range from around 4.8% to 4.9%.