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tokens: has an outer radius of 38 kpc and Bruce Rout found a value of 55 kpc (Bruce 2009 ). Figure 1. Rotation curve of NGC 3198. Figure 1 shows the observed rotation curve of NGC 3198 with two fitting lines: the line “ab” for the radius r r t and the line “bc” for the radius r > r t . The quantities α (the slope of the line ab), β (the slope of the line bc), r t (the turn-off radius), and γ (the intersection point between the line bc and the velocity axis) were taken from the rotation curve and their values are shown below,The flatness characteristic of spiral galaxies is bound with the eccentricity ( k ) calculated from the relation (Binney Tremaine 1987 ). Calculations of the mass and density at different radii of the galaxy NGC 3198 using the above values taken from rotation curve ( α , β , γ , and r t ) and using Equations ( 8 )–( 11 ) were carried out using a computer program constructed for this purpose. The central inner density (at r r t ) calculated for NGC 3198 is found to be constant ( ρ 1 = 0.387 M o pc −3 ), where M o is one solar mass. The density of the outer regions ( r > r t ) of the spiral galaxy NGC 3198 is much less than that of the inner regions. The value of ρ 2 (r) changes from 0.0187 M o kpc −3 at r = 4 kpc to reach 0.00278 M o pc −3 at r = 30 kpc, so the outer density decreases with increasing the distance from the center of the galaxy (Figure 2 ). Figure 2. Outer density as a function of radius. The behavior of the calculated densities of the spiral galaxy NGC 3198 (constant inner density and declining outer density) is consistent with other works (Rubin et al. 1977 ; Binney Tremaine 1987 ; Daod et al. 2014 ). The inner mass (at r r t ) of spiral galaxy NGC 3198 grows up very quickly, as in Figure 3 . The inner mass grows from 1.334 × 10 9 M O at r = 0.1 kpc to a mass of 8.5391 × 10 9 M O at a distance of 4 kpc from the galactic center. Therefore, the mass at the turn-off radius (4 kpc) is more than 10 4 times the mass near the center of the galaxy. Figure 3. Inner mass distribution. The calculated outer mass of NGC 3198 changes from 8.539 × 10 9 M O at the turn-off radius to 5.262 × 10 10 M O at the outer radius ( r g = 30 kpc) of the galaxy. In other words, the mass at the edge of the galaxy is only five times more than that at the turn-off radius. Therefore, the outer mass ( r > r t ) of the galaxy NGC 3198 grows slowly compared to the inner mass, as in Figure 4 . Since outer borders of spiral galaxies extend to a large distance from the visible region. The outer halo mass is significant and it is four times more than the mass of the inner (more dense) regions, which confirm the existence of a massive halo and giving evidence as to the existence of dark matter (Begeman 1989 ). Figure 4. Mass distribution of NGC 3198. The galaxy is usually divided into four regions, the nucleus, the bulge, the disk, and the halo. The mass of these four regions can be calculated if the radius (distance from the center of the galaxy to the