The hydrodynamic instability of the three-dimensional boundary layer on a rotating disk introduces a periodic modulation of the mean flow in the form of stationary cross flow vortices. The instability labeled Type II by Faller occurs first at lower Reynolds number than that of well known Type I instability. Detailed numerical values of the amplification rates, neutral curves and other characteristics of the two instabilities have been calculated over a wide range of parameters. Presented are the neutral stability results concerning the two instability modes by solving the appropriate linear stability equations reformulated not only by considering whole convective terms but also by correcting some errors in the previous stability equations. The present stability results agree with the previously known ones within reasonable limit. Consequently, the flow is found to be always stable for a disturbance whose dimensionless wave number is greater than 0.75. Some spatial amplification contours have been computed for the stationary disturbance wave, whose azimuth angle $varepsilon$= 11.29$^{circ}$ to 15$^{circ}$ and for the moving disturbance wave, whose azimuth angle $varepsilon$ = 12.5$^{circ}$ to 15$^{circ}$. Also, some temporal amplification contours have been computed for the stationary disturbance wave, whose azimuth angle $varepsilon$= 11.29$^{circ}$ to 15$^{circ}$ and for the moving disturbance wave, whose azimuth angle $varepsilon$= 12$^{circ}$ to 15$^{circ}$. The flow instability was observed by using a white titanium tetrachloride gas over rotating disk system. When the numerical results are compared to the present experimental data, the numerical results agree quantitatively, indicating the existence of the selective frequency mechanism.