[HB]

I had done rheology study of a gel and obtained both loss modulus result and storage modulus results as 30 points (30 readings). I need to calculate G`/G" ratio, how can I calculate it, I mean should I calculate it before the yield stress or should I take an average of the 30 readings 

[mjc123]

What are the conditions of your experiment? If you took 30 readings under identical conditions, you can take average values of G' and G'' and take a ratio between them. But be careful, you might have to ignore some points if your sample took a while to equilibrate, or was changing with time.

But you say "should I calculate it before the yield stress?", which implies you are changing conditions through the experiment - whether stress, or strain, or strain rate, or possibly temperature. In this case it makes no sense to average the 30 points. Just calculate the ratio G'/G'' at each point and plot it against your independent variable.

[HB]

I change the stress from 0.1-100 Pa and calculate G' and G'' at 1 Hz (constant frequency)
then I change the frquency from 0.1-100 Hz and calculate G' and G'' at 1 pa (constant stress)

[mjc123]

Then you must not average the 30 readings! Take the ratio at each point, and you can plot it versus stress at 1 Hz, or versus frequency at 1 Pa.