FWHM is not constant, but is a trigonometric function of 2 theta.

Dear Loganathan Kulandaivel ,

in addition to the answers of Dalibor Matýsek and Nilay Maji please resolve the Scherrer equation*), which roughly relates the crystallite size, the peak width, the wavelength and the Bragg angle theta,

you will find:

ß ~ FWHM ~ 1/cos(theta).

For completenes; as a function of 2theta, as mentioned above, you will have:

ß ~ FWHW ~ 1/cos[(2theta)/2]

*) see for example: https://en.wikipedia.org/wiki/Scherrer_equation