Can I convert length direct line fragment from decimal degree to km or meters I tried used IUnitConverter, but in ConvertUnits method, does not provide information about latitude and the result is not correct.
Angles don't convert to distance directly. The length of a line fragment expressed in degrees is pretty much meaningless. You need the start and end point of your line fragment to be able to get the length in a meaningful unit. The IMeasurementTool allows you to construct a geodesically correct line and get the distance if you provide the spatial reference.