You can do this from the ROI Tool ->Options->Report area of ROIs.

I do not want measure ROI class area and do not have any ROI class; I want measure the terrain area which is satellite imageThank you so much

The only tool I know of that can measure area is the ROI tool. So, if there is a specific area of terrain you are interested in (for example, 50-100m elevations), you can make an ROI of this region using the Threshold. Once you have created an ROI of this area, you can then get the area statistics.

Actually, there is another tool. In the ENVI 5.3 toolbar at the top, there is a Mensuration tool. After you select it, also at the top bar, select the polygon annotation type. You can draw a polygon around an area of interest and after you accept the polygon (right click and choose 'Accept'), the annotation will show the area of the polygon and the length of each line segment.

Thank you so much for your kind answer, but I think it is not very accurate! I have got this rule to measure. could you say it is correct or no please?Area sqm= Total count(Row *Column)* image pixels resolution (ex: 5*5) /1000000

Yes, this assumes a rectangular shaped area. For example, in the ENVI sample image qb_boulder_msi, there are 1024x1024 samples and lines and a pixel size of 2.8x2.8. To get the area in square meters: 1024 x 1024 x 2.8 x 2.8 = 8220835.84 then divide to get km.