2018年臺北市長選舉，兩位領先者票數非常接近，極大化了策略投票的誘因，但第三名的候選人仍獲得頗高比例的得票，讓此次選舉有效候選人數成為24年來新高。本研究使用「台灣選舉與民主化調查」2018年臺北市長選舉面訪案(n=1130)，分析這個特殊個案中選民的策略投票行為。研究結果發現：(1)資料中大約有9.9%的選民投給第二偏好。(2)選民策略投票與否，取決於對第一偏好候選人的偏好程度高低，或者第一、第二偏好候選人偏好程度的差異。當覺得第一、第二偏好差不多、或者第一偏好又不夠好時，就越可能投給第二偏好。(3)選民策略投票與否，與第一偏好當選機率無關，完全取決於偏好程度本身。(4)在前兩點的作用下，姚文智支持者的策略投票比例並沒有比另兩位候選人的支持者為高。即使臺北市選民認為自己支持的對象不會當選，但對其評價遠大於其他候選人，因此最後仍誠實投票，而無視當選機率，促成了本次選舉的得票分布。

In the 2018 Taipei City Mayoral Election, voters were highly motivated to consider voting strategically, given that there was a very close race between the two frontrunners. Surprisingly, the third-place candidate still received a considerably large proportion of the votes in this election, which caused the effective number of viable candidates to reach a twenty-four-year high in Taipei City. In this article, we utilize Taiwan’s Election and Democratization Study 2018A (n=1130) to investigate why voters chose to, and chose not to, vote strategically. The results show that: (1) about 9.9% of respondents voted for the second-preferred candidate; (2) the decision to vote strategically was mainly driven by the respondents’ approval of the most-preferred candidate and the difference of approval between the most- and second-preferred candidates. Voters are much more likely to choose the second-preferred candidate if the most-preferred candidate is not perceived to be good enough, while the difference between the most- and second-preferred candidates is close. (3) The candidates’ perceived chance of winning has no impact on the voters’ decision to vote strategically; (4) owing to the two former factors, the tendency of strategic voting is similar across the supporters of the three major candidates. Hence, voters may perceive that their preferred candidate will not win, but also think that their preferred candidate is far better than the other two frontrunners, so they ignore the chances of winning and still vote sincerely. Such a strong preference gap may have contributed to the unique ballot distribution in the 2018 Taipei City Mayoral Election.