Amplification of acoustic in-plane phonons due to an external temperature gradient <$E ( grad T)> in single-layer graphene (SLG) was studied theoretically. The threshold temperature gradient <$E ( grad T) sub 0 sup g> and the threshold voltage <$E (V sub T ) sub 0 sup g> in SLG were evaluated. For T = 77 K, the calculated value for <$E ( grad T) sub 0 sup g> 746,8 K/cm and <$E (V sub T ) sub 0 sup g~=~6,6> mV. The calculationwas done in the hypersound regime. Further, the dependence of the normalized amplification (<$E GAMMA "/" GAMMA sub 0>) on the frequency <$E omega sub q> and <$E grad T "/" T> were evaluated numerically and presented graphically. The calculated threshold temperature gradient <$E ( grad T) sub 0 sup g> for SLG was higher than that obtained for homogeneous semiconductors <$E (n-InSb)( grad T) sub 0 sup hom~symbol Ы~10 sup 3> K/cm, superlattices <$E ( grad T) sub 0 sup SL~=~384> K/cm, and cylindrical quantum wire <$E ( grad T) sub 0 sup cqw~symbol Ы~10 sup 2> K/cm. This makes SLG a much better material for thermoelectric phonon amplification.