This study discusses the following two problems using experimental data from modern Hebrew: the relationship between the duration of vowels and the concepts of so-called “long vowel”, “short vowel” and “mobile sheva” as phonological vowel signs and the relationship be-tween the syllable on which stress falls and the duration of vowels within a word. The duration of vowels in normal speech was measured in twentytwo segolate nouns and thirteen “QAL” verbs with the root “KTV”. The informant was a female native Israeli. The test words were presented separately and with a carrier sentence. The speech sound recorded in an anechoic room was analyzed using a soundspectrograph, the speech wave form and the SPSS program on VAX-11/780 system. The difference in the length of vowels was examined via t-tests. It was found that the duration of vowels was longer in stressed syllables, but was not always related to the phonological vowel signs called “long vowel” or “short vowel”. The duration of “mobile sheva” was not shorter than the other vowels when it could be regarded as a vowel on the spectrogram. In some cases, no duration with vowel quality was observed for “mobile sheva”. It became clear that duration of vowel was controlled by the stressed syllable of the word.