In computer network system, the diameter always be represented the worst delay in the communication between any two nodes, it is an important factor to measure transmission efficiency, and the average diameter is the average delay between any two nodes, can it be measured transmission efficiency? The minimum distance diagram of a directed double-loop network is always an L-shaped tile, which can be described with four geometric parameters: a,b,p and q. According to the L-shaped tile, we proved that the average diameter of directed double-loop network was also up to four geometric parameters: a,b,p and q, we provided two algorithms to compute them, and simulated the relationship between average diameter and diameter. Simulation indicates that the distribution of the diameter and the average diameter are all axis-symmetrical figure; the average diameter approaches to a half of the diameter at the same point; the diameter does not always obtain minimum value, when the average diameter obtains minimum value. Finally, we verified that the lower bounds about the diameter and the average diameter of the directed double-loop networks provided by Wong and Coppersmith were correct, and confirmed Fang et al.'s suspicions. So we get that the average diameter also cam be measured transmission efficiency in computer network system .