I guess you're talking about waveguides arent you? Microstrips dont have cutoff frequency for their fundamental mode.

Regardless, I advise you the use of some good field solver(e.g. CST Microwave Studio) to get all these parameters computed.  Modes' cutoffs are easily computed with them, for waveguides/microstrip lines etc.

Please take a look at the paper called "A Stepwise Nicolson–Ross–Weir-Based Material Parameter Extraction Method", where the microstrip case is considered.

Marcelo is right, you don't have a cutoff frequency for microstrip lines, the original method was developed for waveguides.

cutoff frequency define for waveguide , not microstrip .Because in microstrip the TEM mod Excite and TEM mode don't have any cutoff frequency

I suggesst you read the paper,  "A Stepwise Nicolson–Ross–Weir-Based Material Parameter Extraction Method",  cutoff applies to waveguides, the microstrip case is considered in this paper. Try your simulations using HFSS, observe the scattering parameters, your best approach is this.

Why do you know cut-off in a microstrip anyway? IMHO you are trying to find out frequency for resonant mode? 

Normally and theoretically you don't have cutoff frequency associated with micro-strips. They ideally operate in their TE (Quasi) mode however for practical purposes and to prevent higher-order transmission modes, there is a limit which defines line physical parameters. For example the thickness of the micro-strip line needs to be greater than Lambda-g/10 or in other words you should limit the thickness of your micro-strip substrate to 10% of a wavelength. 

The next modes are shown in the HFSS/CST/FEKO simulations, just set the number of modes to 2 in the wave port.

Pozar's book also have some formulas for that limits on the next modes for microstrips.