Thanks for the answer, in terms of digital sensor , do you mean the output of the sensor is digital ? Because most of the sensors I've looked at is analogue ones and the output is about max 500mV, it can't be recognize isn't it? So another question is that is there any block in market can transmit 0-500mV to 0-3.3V?

Yes I mean digital output. There are many digital temperature sensors out there. And unless you have a specific reason to go analog, I would just use a digital one.

If you go analog and it has a max of 500mV, you can directly interface it with an AnalogIn pin. Its maximum input is 3.3V, but less is no problem (although it costs accuracy).

That's nice, since you've mentioned the accuracy, can I ask how much the accuracy would be approximately if I use a 500mV temperature sensor? And is there any way to save the accuracy? By the way, I'm trying to drive a SSR, would the output of the micro controller be able to drive a 14uA load current SSR?

It is a 12-bit ADC over 3.3V, so 3.3V / 4095 is roughly 1mV accuracy. Now on the mbed it isn't that good due to some routing stuff, so lets say 2mV accuracy, it depends on your temperature sensor how much degrees that is.

For the SSR, if 3.3V is enough for it, then that isn't a problem, 14uA is very little.