The smallest mammal in the world is the bumblebee bat which lives along the River Kwai in western Thailand. An endangered species, the bumblebee bat is not much larger than an actual bumblebee (1.2 in) and weighs less than a penny (0.07 oz). Of course, there are several mammals similar in size to the bumblebee bat, and the designation of which one is actually the smallest can sometimes be contentious. Another animal which is sometimes awarded the title of "world's smallest mammal" is Savi's pygmy shrew, a tiny insectivore small enough to run through the tunnels of large earthworms. Each day, this pygmy shrew consumes insects and spiders up to 2 to 4 times its own body weight.

1) "The endangered Kitti's Hog-nosed Bat (bumblebee bat; Craseonycteris thonglongyai) of Thailand is the smallest mammal, at 30-40 mm in length and 1.5 to 2 g in weight, about the weight of a Canadian or U.S. dime. The Etruscan Shrew is the smallest mammal by mass, though it exceeds the Bumblebee Bat in skull size." Source and further information: http://en.wikipedia.org/wiki/Smallest_organisms#Smallest_mammal 2) "Kitti's Hog-nosed Bat (Craseonycteris thonglongyai), also known as the bumblebee bat, is an endangered species of bat and the only member of the family Craseonycteridae. It occurs in western Thailand and southeast Burma, where it occupies limestone caves along rivers. Kitti's Hog-nosed Bat is the smallest species of bat and one of the world's smallest mammals." "Kitti's Hog-nosed Bat is about 29–33 millimetres (1.14–1.30 in) in length and 2 grams (0.07 oz) in mass), hence the common name of "bumblebee bat". It is the smallest species of bat and may be the world's smallest mammal, depending on how size is defined. The main competitors for the title are small shrews; in particular, the Etruscan shrew may be lighter (1.2–2.7 g) (0.042-0.095 oz) but is longer (36–53 mm head-and-body) (1.42-2.09 in)." Source and further information: http://en.wikipedia.org/wiki/Bumblebee_Bat 3) "The Etruscan Shrew (Suncus etruscus) , also known as the Etruscan Pygmy Shrew or the White-toothed Pygmy Shrew is the smallest known mammal by mass, weighing only about 2 grams, although the smallest known mammal by skull size is the Bumblebee Bat. The smallest mature specimens of this shrew are thought to weigh about 1.3 grams and measure 36 mm (1.42 inches) long. This small mammal is 60 mm from the tip of the nose to the base of the tail; the tail is about 40 mm long. This shrew has a lifespan of 15 months. The Etruscan Shrew inhabits forests and brush areas between Southern Asia and Southern Europe. A forager, the Etruscan Shrew subsists largely upon insects." Source and further information: http://en.wikipedia.org/wiki/Etruscan_shrew