Ad-hoc wireless network with a huge amount of static or mobile sensors is called wireless sensor network (WSN). The sensors collaborate to sense, collect and process the raw information of the phenomenon in sensing area (in-network), and transmit the processed information to the observers. Robustness, fault tolerance, and furthermore are prime factors to be consider before designing wireless sensor network. It is theoretically proved through math modeling, theoretical analysis and formula deducting that sensor deployments in the form of equilateral triangle, as a rule, are better than those in the form of square, hexagon and octagon. The theoretical analysis formula has given that minimum number of sensor nodes is demanded in a given sensor field if the field is covered fully and seamlessly and efficient coverage area ratio decrease with increase of sensor nodes comparing to same grid.