This thesis attempts to clarify the role that tariff policy has played and its currently playing in affecting the economic development of the least developed countries (LDCs). The goals of this thesis are threefold. First, the extent, speed, timing of trade liberalization in the LDCs is analysed using new information provided directly by the Trade Ministries in the LDCs. A panel probit econometric analysis is used to identify the probability of aid flows influencing the timing of trade liberalization. It is shown that aid flows have played a non-negligible role in affecting the timing of trade liberalization. Second, a new hand-gathered tariff database is constructed at 4-digit level for 18 LDCs from 1970s to 2001/2 to fill in the LDCs' statistical gap. Lastly, the thesis studies the variations of the components of import demand in the LDCs and analyses the effect of aid flows and tariff policy on imports