As we sometimes say, "Apples and Oranges". They are different. The two types of techniques and detection modes have absolutely nothing in common with each other so there is no reason to expect similar results at all. GC/MSD (I will assume you are using EI, but you did not say) is completely dependent on the method conditions used and injector, detector settings chosen. The high heat in GC also degrades the sample, changing the results. Since MS relies on ionization alone, it is a poor choice for quantitation too. *For better results, use HPLC with DAD instead (or LC-DAD-MS, for the best results).

Tetrahydrocannabinol (THC) has an excellent chromophore so UV/VIS would provide a much better and reliable signal (based on simple absorbance) for quantitative work. Esp when a calibration curve is generated using standards. *GC/MS is useful in identifying alone, but the two separate techniques can be used in an orthogonal manner to provide useful data. You might also want to use GC/FID/MS for even more data, esp if not derivatizing the sample to obtain total THC and THCA amts by LC.

Cannabis plant contains much higher THC-A than THC (higher ratio of THC-A/THC).

In GC analysis, the high temperature (> 150oC) in GC injector port  decarboxylates THC-A to THC and therefore you have apparent higher THC concentration as compared to DAD analysis. To obtain an accurate THC concentration, LC is a preferable method.

Thanks Kong M Li!! so, if I want to extract only thc, can I decarboxylate all the THCA? And How I can do that?

Thanks sir

Not sure EtOH is the best choice here.  THC is a fat-soluble (ie lipophilic) drug.  Perhaps a more polar solvent would be more effective?  Also are you analyzing the extract as is or are you doing any sort of SPE work-up?

The decarboxylation of THC-A to THC might not be completed in GC analysis. Therefore LC is best method to obtain the accurate concentrations of THC-A. Another way of expressing the potency of the cannabis plant is total THC  which equals to THC + THC-A.

Thanks a lot to all of you, in particular to Kong M Li.

Apples and Pears are correct; UV-VIS DAD least sensitive MS most sensitive whether use GC or LC. Use MS for your analysis; set up your method development; Validate and test your solutions at range of concentrations; kept at different temperatures. MS better then Diode Array (UV-Vis) Detection.