We will use immersed surfaces to study Seifert fibered surgery on Montesinos knots, and show that if 1/q1(-1) vertical bar 1/q2(-1) vertical bar 1/q3(-1) <= 1, then a Montesinos knot K(p1/q1, p2/q2, p3/q3) admits no atoroidal Seifert fibered surgery.