We investigate the joint actuator-sensor design problem for stochastic linear control systems. Specifically, we address the problem of identifying a pair of sensor and actuator which gives rise to the minimum expected value of a quadratic cost. It is well known that for the linear-quadratic-Gaussian (LQG) control problem, the optimal feedback control law can be obtained via the celebrated separation principle. Moreover, if the system is stabilizable and detectable, then the infinitehorizon time-averaged cost exists. But such a cost depends on the placements of the sensor and the actuator. We formulate in the paper the optimization problem about minimizing the time-averaged cost over admissible pairs of actuator and sensor under the constraint that their Euclidean norms are fixed. The problem is non-convex and is in general difficult to solve. We obtain in the paper a gradient descent algorithm (over the set of admissible pairs) which minimizes the time-averaged cost. Moreover, we show that the algorithm can lead to a unique local (and hence global) minimum point under certain special conditions. * X. Chen is with the Lemma 2. For fixed b and c with (A, b) stabilizable and (A, c) detectable, we have ∂Φ(r, s)/∂r ≤ 0, ∂Φ(r, s)/∂s ≤ 0.The inequalities are strict if K ′ (r) < 0 and Σ ′ (s) < 0.Proof. We focus only on the proof for ∂Φ(r, s)/∂r ≤ 0. By symmetry, the same argument can be applied to establish ∂Φ(r, s)/∂s ≤ 0. For convenience, we let K ′ (r) := dK(r)/dr. We obtain by computation ∂Φ(r, s)/∂r = tr((AΣ(s) + Σ(s)A ⊤ + I)K ′ (r)) = s tr(Σ(s)cc ⊤ Σ(s)K ′ (r)) ≤ 0where the second equality comes from (6), and the last inequality comes from the fact that tr(P Q) ≤ 0 for P ≥ 0 and Q ≤ 0. Here, P := Σ(s)cc ⊤ Σ(s) and Q := K ′ (r). The