Hello Jin,
I recommend you use something similar to following workflow:
1. Export the coverage to a shapefile or feature class
2. Use the project tool (Data Management > Projections and Transformations > Feature) to reproject your data to a Projected Coordinate System (not Geographic Coordinate System)
3. If you exported to a feature class in a geodatabase, shape length will be automatically generated for you using the same units as your coordinate system (likely feet or meters)
4. If you are using a shapefile, open the attribute table, click table options, and select Add Field
5. Name your field Length and make it type Double and click OK
6. Right click your new field header and select Calculate Geometry
7. Click Yes if you receive a message asking if you want to do this outside of an edit session
8. Select that you want to calculate length
9. Select the unit that you want to use and click OK