Temperature does not go below 0

[Scampy]

Temperature goes above ZERO.Positive temperature is displayed correctly.When i keep thermistor in freezeo then temperature does't go below ZERO.

Try some basic measurements. remove the thermistor from the board and place it in your freezer with wires connected. Measure the resistance using a digital volt meter and note it down, together with the temperature. Repeat the measurement when the thermistor is placed in ice water, and again when under a lamp or something that you can measure the temperature with. This may help you calibrate the routine better.

One possible issue could be that your chosen thermistor is positive only, ie at zero degrees C it's resistance is zero ohms (or as near to as dam it).

[Charlie]

Without the datasheet for your device, it's impossible to resolve it for you, but here's the data for a typical device:
http://www.arroyoinstruments.com/thermistors.html It has a table of values vs temperature. So, at -10C it's about 55Kohm, and at +50C it's about 3.6K ohms. So now you need to turn that into a voltage either using a resistor divider, or a current source. This is simple math, easy to do with Excel, and you can create a list of voltages for each degree or half degree or whatever you like. Then use those voltage values to lookup the corresponding temperature.