We determine the stability/instability of the tangent bundles of the Fano varieties in a certain class of two orbit varieties, which are classified by Pasquier in 2009. As a consequence, we show that some of these varieties admit unstable tangent bundles, which disproves a conjecture on stability of tangent bundles of Fano manifolds. 0.1. A Fano manifold X is, by definition, a smooth projective variety X whose anti-canonical divisor −K X is ample. From a differential geometric viewpoint, it is very important to detect which Fano manifolds admit Kähler-Einstein metrics and, after the celebrated works [Tia97,Don02,Ber16,CDS15a,CDS15b,CDS15c,Tia15], it has been accomplished that the existence of a Kähler-Einstein metric on a given Fano manifold X is equivalent to a purely algebraic stability condition for X, called K-polystability.As is well-known, not every Fano manifold admits a Kähler-Einstein metric. For example, Matsushima proved that the existence of a Kähler-Einstein metric on a Fano manifold X implies the reductivity of the automorphism group of X [Mat57]. The automorphism group of a Fano manifold X is, however, not always reductive. Also it is usually very difficult to determine whether or not a given Fano manifold X admits a Kähler-Einstein metric, though the existence of such a metric is rephrased by the K-polystability of the Fano manifold X in question.Therefore it would be useful to study several variants of stability conditions on Fano manifolds. As such a variant, stability of tangent bundles (in the sense of Mumford-Takemoto) has attracted several attention of researchers. By the Kobayashi-Hitchin correspondence, the polystability of the tangent bundle is equivalent to the existence of a Hermitian-Einstein metric on the bundle [Kob82, Lüb83, Don85, UY86, Don87], and hence the polystability of the tangent bundle is weaker than the existence of a Kähler-Einstein metric. Also, thanks to its simplicity, the stability of the tangent bundle is rather easy to handle in the framework of algebraic geometry. Moreover it is expected that, for a given Fano manifold X, the (in)stability of the tangent bundle reflects very well the geometry of X. For example, a folklore conjecture 1 claims the following: Conjecture 0.1 (Stability of tangent bundles). Let X be a Fano manifold. Assume that the Picard number ρ X of X is one. Then the tangent bundle Θ X is (semi)stable. Conjecture 0.1 has been confirmed in the following cases:(1) r X = 1 [Rei78, Theorem 3], where r X is the Fano index of X;