Introduction  Affected by typical micro-topography， the wind speed of overhead transmission lines will increase. This paper aims to study the influence of micro-topography on the force of transmission tower.

  Method  To investigate the parameters and the distribution law of topographic factors， we compared topographic factors of two typical mountain topographies for the crest and the escarpment among China， America and Australia codes. The force analysis of transmission towers were carried out with different codes， different route directions and different locations.

  Result  The results we obtained demonstrate that the distribution law of topographic factor of the three codes is basically the same， and the degree of influence by slope， horizontal distance and height is different， the force of main member of tower body is the maximum when the transmission line runs along the top of the mountain， the influence of topographic factor on the force of tower is greater than wind speed increased by 10%.

  Conclusion  It is suggested that the transmission line should cross the top of the mountain perpendicularly as much as possible， and the influence of topographic factor should be considered for transmission towers located in two typical mountain topographies.