During cardiac development, the growth, remodeling and morphogenesis of embryonic hearts are closely linked to hemodynamic forces. An understanding of the interaction mechanism between hemodynamic forces and heart development is important for the early diagnosis and treatment of various congenital defects. The myocardial wall strain (MWS) in embryonic heart is a critical parameter for quantifying the mechanical properties of cardiac tissues. Here, we focus on the radial strain which is defined as the change of the myocardial wall thickness. An effective measurement of MWS is conductive to studies of embryonic heart development. Chick embryo is a popular animal model used for studing the cardiac development due to the similarity of cardiac development between the human heart and the chick heart at early developmental stages and its easy access. Although various imaging methods have been proposed, there still remain significant challenges to imaging of early stage chick embryo heart because it is small in size and beats fast. Optical coherence tomography (OCT) is a non-contact three-dimensional imaging modality with high spatial and temporal resolution which has been widely used for imaging the biological tissue. In this paper, we describe a method to measure in vivo MWS of chicken embryonic hearts with a high speed spectral domain OCT(SDOCT) system worked at 1310 nm. We perform four-dimensional (4D) (x, y, z, t) scanning on the outflow tract (OFT) of chick embryonic hearts in a non-gated way. The transient states of the OFT are extracted from the 4D data by using the beating synchronization algorithm. The OFT center line can be achieved by image processing. Assuming that the blood flow is parallel to the center line in the blood vessel, we calculate the Doppler angle of blood flow from the OFT center line. In a certain OFT cross-section, the OFT myocardial wall (inner and external borders) is segmented from the OCT images with a semi-automatic boundary-detection algorithm. Then, the myocardial wall thickness is calculated from the Doppler angle, area and sum of inner and external radii of the segmented myocardial wall. The radial strain is obtained by calculating the myocardial wall thickness variation. Previous methods calculated the myocardial wall thickness by directly subtracting inner and external radii. The measured result may be deteriorated by insufficient resolution of the system since the myocardial wall of OFT is very thin. The present method can solve this problem by calculating the thickness through using the sum of the radii instead of the subtraction. The experimental results on embryonic chick hearts demonstrate that the proposed method can measure the MWS of OFT along arbitrary orientation and it is a useful tool for studying the biomechanical characteristics of embryonic hearts.