Battery module assembly of improved reliability and battery pack employed with the same

A battery module assembly having one or more plate-shaped battery cells, comprising:

• two or more battery modules having cell units, each of which comprises one or more battery cells, electrically connected to one another;
• a pair of an upper case and a lower case to surround outer circumferences of the battery modules such that one surface of each of the outermost battery modules is exposed outside in a state in which the battery modules are erected on their sides;
• one or more first fastening holes vertically formed at a top of the upper case and a bottom of the lower case in a direction in which first fastening holes face each other to fasten and fix the upper case and the lower case;
• and
• two or more second fastening holes horizontally formed at a front and/or a rear of the upper case and a front and/or a rear of the lower case to position or fix the battery module assembly.