Most of present hardwares such as CPU, Mainboard, GPU or Harddisk come with temperature sensors. Unfortunately, driver or software that come with hardware always only working on Windows. So we will use Sensors Applet to monitor hardware temperature on Ubuntu.
We can install Sensors Applet via Terminal with command
sudo apt-get install sensors-applet
During installation there will be dialog. You can choose default options.
After finish installation you can add Sensors Applet Panel by right click on empty panel then choose Add to Panel...
Choosing Hardware Sensors Monitor then click Add button.
So it will show temperature of CPU, Mainboard and GPU (This is depend on your machine)
If you want to setting how to show temperature. You can go to Sensors Applet Preferences by right click on Sensors Applet Panel then choose Preferences
There will be dialog to set temperature interface.
You can choose hardware you want to monitor in Sensors Tab