You pose one of the most fundamental philosophical questions in ecology, Takele. A large portion of your decision making process should come down to your hypotheses and research objectives. If you are studying rare plants or trying to detect early stages of bio-invasion, then as Abhishek advises, Whittakers are the way to go. However it is important to understand that ocular estimates of percent cover become increasingly inaccurate with either very low or relatively high species prevalence. Thus if it is important for you to quantify your results within fairly narrow confidence interval limits and/or low alpha errors, I would recommend the point/line intercept method. But, you will miss a number of very rare species using that method. If your goal is to catalog every last species present, then an exhaustive floristic survey is warranted. However, if you endeavor to quantify total diversity, then I would not be overly concerned about missing some very rare species, as their extremely low relative percent covers will be negligible concerning total diversity.

there are two methods for measurement of species richness. transect method and point count method. divide the habitat according to area. I will also go for Whittakers methods as like other scientist...

Dear Geta

The process of Measurement of the Species richness entire depends on your choice and demand.  if you want to measure the species richness of a rare and endangered species in the range lands or grasslands, then it is better to follow the modified Modified-Whittaker methods as Abhishek Raj suggest in his answer as the second option.  but if you want to measure the numbers and the diversity of the species in a range lands or a grass land then the line transects methods is the most effective one.

With Regards

Sunit

Shannon Weiner and Stress indices are good indicators

I think transect method is suitable for you according to you study area (rangeland or grassland)

I also suggest the Shannon-Wiener index, as well as an index on eveness to have a more complete analysis!

I agree with those who proposed Shannon's information. Even though this measure is not easy to interpret, it is used traditionally and it can be at least compared across the studies which uses this index. Shannon’s information as well as the reciprocal of Simpsons dominance already incorporate Evenness, so the latter will be more informative if considered together with the species richness, which is the most meaningful index of biodiversity. You can use EstimateS (software free to download) for appropriate analyses and assessment of species richness (rarefaction, Chao’s estimator, etc.).

For sampling, transects of random directions or small squares placed at random can work if they cover ca. 10% of the studied sites…