You can read the data into IDL using the GetData method. An example how this can be achieved can be found using the following help page:
https://www.harrisgeospatial.com/docs/enviRaster__GetData.html

You can then use the WRITE_TIFF routine to generate a geoTIFF file. An example of how to write a GeoTIFF file can be found on the following help page (Example 4):
https://www.harrisgeospatial.com/docs/write_tiff.html

Alternatively, you can use the ChipToFile ENVI method to generate a geoTIFF from an ENVI view:
https://www.harrisgeospatial.com/docs/ENVIView__ChipToFile.html

-David
Harris GS

Hi, David,

Thank you for your response. I have followed the documents you posted to convert ENVI file to Geotiff format. A key step for the conversion was to set the GeoTiff parameter of WRITE_TIFF API. Few of the parameter was come from the *.tfw file associated with the Geotif file that I manually convrted in ENVI. The following codes can convert ENVI file to corresponding Geotiff file. However, I am not sure the parameter is suitable for all files in different regions, since I can not test on all of them. Can you comment on this?

...
ENVI_FILE_OPEN, file, R_FID=fId
ENVI_FILE_QUERY,fId,DIMS=dims,NS=ns,NL=nl,NB=nb,BNAMES=bandNames,FILE_TYPE=fileType,DATA_TYPE=datType;
mapInfo = ENVI_GET_MAP_INFO(FID = fId);
prj = ENVI_GET_PROJECTION(FID=fId,PIXEL_SIZE=ps,UNITS=unit);
data = INDGEN(nb,ns,nl);
FOR j = 0L, nb-1 DO BEGIN ; Multiple bands
data[j,*,*] = ENVI_GET_DATA(FID=fId,POS=j,DIMS=dims);
ENDFOR

g_tags = { $
ModelPixelScaleTag: [ps[0], ps[1], 0d], $
ModelTiepointTag:[0.00000000000000000, 0.00000000000000000, 0.00000000000000000, mapInfo.MC[2], mapInfo.MC[3], 0.00000000000000000],$
GTModelTypeGeoKey: 2, $ ; (ModelTypeProjected)
GTRasterTypeGeoKey: 1, $ ; (RasterPixelIsArea)
GeographicTypeGeoKey: 4326, $ ; (GCS_NAD27)
ProjectionGeoKey: 32767, $ ; (user-defined)
ProjLinearUnitsGeoKey: 9001, $ ; (Linear_Meter)
GEOGLINEARUNITSGEOKEY:9102 $
}
fileBaseName = FileBaseName(fileName);
outFile = 'D:\PALSAR-2-V2\Geotiff\'+fileBaseName+'.tif';
WRITE_TIFF,outFile,data,/LONG,GEOTIFF=g_tags;
; Delete memory file
ENVI_FILE_MNG,ID=fId,/DELETE;

If you have ENVI, then you could also use ENVI's API to export to geoTIFF. You could use ENVI Classic's ENVI_OUTPUT_TO_EXTERNAL_FORMAT:

https://www.harrisgeospatial.com/docs/ENVI_OUTPUT_TO_EXTERNAL_FORMAT.html

or use the new API to export the raster object:
https://www.harrisgeospatial.com/docs/enviraster__export.html

These routines will export the appropriate geoTIFF tags to the file. However not all metadata that is supported for an ENVI raster is supported in geoTIFF. For example, metadata such as acquisition time, RPCs, and other custom items are not supported in geotiff tags.