In this paper, design and developing of an optical spectra measuring instrument based on a diffraction grating and a CMOS webcam sensor – without using slit – has been described. The instrument is a slit free spectrometer. This cost effective spectrometer achieves real accuracy of 0.06 nm, suitable for resolving many spectral lines. This system has been tested for resolving the sodium doublet lines at 589 nm. The input of the instrument is a single mode optical fiber and the output of the instrument is a USB port. The software of the system is based on the LabVIEW program.