You can measure current-voltage characteristics and extract resistivity from their linear parts. From resistivity you can estimate conductivity. See an example for N-doped graphene in the attached paper.

Dear Habib,

I think that depends largely on how large your samples are and how they are supported. If you have only snippets of graphene a few 100 nm across, direct current-voltage characteristics might be difficult. You can try optical methods, if the graphene is on a surface, or conductive AFM, if you can calibrate it properly.

However, if you do have large samples, I agree with Sergei, that current-voltage direct measurements (e.g. 4-point method), might be the best solution.

Good luck!

You can transfer it on Si/SiO2 substrate and then with photolithography make TLM structures ( Transmission Line measurements ). 

Transfer graphene layer between two metal electrodes. Measure I-V characteristics, estimate resistance.

As mentioned by Mona, measure resistance through TLM.

You can not directly probe graphene sheet using four-point-probe technique, as thiclness of graphene is very small.

How large individual RGO sheets ? or How lateral size of your samples?

To solve the problem, I need more information.

I think that you need photolithography ( lateral size > 5 um) or

ebeam lithography (lateral size

I don`t know.I`ve synthsised GO by modified Hummer method.

According to my experience with modified Hummer's method, your graphene will be micrometer-sized at best. You will probably have a mixture of single-layer graphene and 2-5 layers of graphene stacked on top of each other.

You then have powdered samples. You can measure the electricall resistance of yoir powders putting them into a teflon/plastic piston with metall electrodes annd press the  powderbewteen them  with a fixed load and then measure resistance and or iV with a good elecrromeeter. You need to let the system to stabilize some time (measure resisstance vs time until stable values are obtained).. we get stalble reading in one hour. 

You can use EIS-Electrochemical Impedance Spectroscopy- or four probe measurement.