I monitor the reading of sensor from PC. I notice something strange that when I slowly rotate the sensor against x axis, the reading from x axis goes from 0 and increasing slowly, which is normal. but when the reading hits ard 20g, the next reading becomes 0, which is not normal. the reading should keep increasing until ard 58 and then decreasing to 0. what could be the problem?

 

You're not really making sense.
The LIS302DL has a range of +/-2g or +/-8g, depending upon the configuration register FS; if bit FS=0, the range is +/-2g, if 1, +/-8g.

The output for each axis is a 2's complement word (two bytes, or 7 bits + sign). This would give the device output a range of -128 to 127. Scaling (2g, 8g) depends upon the FS bit.

 

What's stored in FS? Or are you using software that allows you to select the 2g or 8g range?

If the axis you're looking at is oriented 90° from the vertical, it should be reading somewhere near zero.

If you wrote your own software for it - are you certain that you didn't write to any of the "reserved" register areas? If so, you may have destroyed the factory calibration values.