Audacity Forum

When the stereo recording is progressing using the line-in input the bars of left and right channel repectively are different most of the time; however the graphic of both tracks are identical, I can confirm the recording was mono when I listen the exported wave file. Any help is appriciated.

The difference in the left and right meter bars may be just because of the slow refresh rate of the meters.
You have given very little information, but my guess is that you are recording a mono signal (is the recording input on your sound card stereo?)

I think my line-in is stereo as the left and right channel bars are different, they are the same when I used the mic input.

If Audacity is receiving stereo data and it is recording on 2 channels, then it will (must) record in stereo. There is something that you are not telling us.